diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..bf750e9
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,11 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+charset = utf-8
+max_line_length = 80
+
+[{*.yml, *.yaml}]
+indent_style = space
+indent_size = 2
diff --git a/CHANGES.md b/CHANGES.md
index f31b81a..f1cecf9 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -10,6 +10,9 @@ To be released.
 
  -  The `uri` type has completly gone; use `url` instead.
     [[#126], [#281] by Jonghun Park]
+ -  Added [`@numeric-constraints`](docs/annotation.md#numeric-constraints)
+    annotation to constraint the range of unboxed types' values.
+    [[#206], [#271] by Seunghun Lee]
 
 ### Docs target
 
@@ -57,7 +60,9 @@ To be released.
     (from [spoqa/nirum](https://hub.docker.com/r/spoqa/nirum/)).
 
 [#126]: https://github.com/nirum-lang/nirum/issues/126
+[#206]: https://github.com/nirum-lang/nirum/issues/206
 [#225]: https://github.com/nirum-lang/nirum/issues/225
+[#271]: https://github.com/nirum-lang/nirum/pull/271
 [#281]: https://github.com/nirum-lang/nirum/pull/281
 [#283]: https://github.com/spoqa/nirum/pull/283
 [#297]: https://github.com/nirum-lang/nirum/issues/297
diff --git a/docs/annotation.md b/docs/annotation.md
index a8a4ace..ed0b1a3 100644
--- a/docs/annotation.md
+++ b/docs/annotation.md
@@ -129,3 +129,32 @@ class FileNotFound(FileError):
 class FileNotReadable(FileError):
     ...
 ~~~~~~~~
+
+### `@numeric-constraints`                                {#numeric-constraints}
+
+`@numeric-constraints` annotation constrains the range of unboxed types' values.
+Currently, available annotation arguments are below:
+
+`min`
+:   Minimum input value; inclusive.
+
+`max`
+:   Maximum input value; inclusive.
+
+For example, the following first Nirum code is compiled to the second Python
+code:
+
+~~~~~~~~ nirum
+@numeric-constraints(min=1, max=12)
+unboxed month (int32);
+~~~~~~~~
+
+~~~~~~~~ python
+class Month:
+    def __init__(self, value: int) -> None:
+        if not value <= 12:
+            raise ValueError("value is greater than 12")
+        if not value >= 1:
+            raise ValueError("value is less than 1")
+        ...
+ ~~~~~~~~
diff --git a/src/Nirum/Targets/Python.hs b/src/Nirum/Targets/Python.hs
index 6d1e5c4..4ccecf5 100644
--- a/src/Nirum/Targets/Python.hs
+++ b/src/Nirum/Targets/Python.hs
@@ -18,7 +18,7 @@ module Nirum.Targets.Python
 import Control.Monad (forM)
 import Control.Monad.State (modify)
 import qualified Data.List as L
-import Data.Maybe (catMaybes, fromMaybe)
+import Data.Maybe (catMaybes, fromMaybe, isJust)
 import GHC.Exts (IsList (toList))
 
 import qualified Data.ByteString.Lazy
@@ -685,12 +685,33 @@ compileTypeDeclaration src d@TypeDeclaration { typename = typename'
     |]
 compileTypeDeclaration src d@TypeDeclaration { typename = typename'
                                              , type' = UnboxedType itype
+                                             , typeAnnotations = annots
                                              } = do
     let className = toClassName' typename'
     itypeExpr <- compileTypeExpression' src (Just itype)
     insertStandardImport "typing"
     pyVer <- getPythonVersion
-    Validator typePred valueValidators' <- compileValidator' src itype "value"
+    Validator typePred valueValidatorsProto <-
+        compileValidator' src itype "value"
+    valueValidators' <- case A.lookup "numeric-constraints" annots of
+        Just A.Annotation { A.arguments = args } -> do
+            let constraintValidators =
+                    [ case (name', value) of
+                        ("min", Integer v) ->
+                            Just $ ValueValidator
+                                       [qq|value >= ($v)|]
+                                       [qq|value is less than $v|]
+                        ("max", Integer v) ->
+                            Just $ ValueValidator
+                                       [qq|value <= ($v)|]
+                                       [qq|value is greater than $v|]
+                        _ -> Nothing
+                    | (name', value) <- toList args
+                    ]
+            if all isJust constraintValidators
+            then return $ catMaybes constraintValidators
+            else fail "Unsupported arguments on @numeric-constraints"
+        Nothing -> return valueValidatorsProto
     deserializer <- compileDeserializer' src itype "value" "rv" "on_error"
     defaultErrorHandler <- defaultDeserializerErrorHandler
     return [compileText|
diff --git a/test/Nirum/Targets/PythonSpec.hs b/test/Nirum/Targets/PythonSpec.hs
index 58f968a..0189c4e 100644
--- a/test/Nirum/Targets/PythonSpec.hs
+++ b/test/Nirum/Targets/PythonSpec.hs
@@ -3,11 +3,15 @@
 module Nirum.Targets.PythonSpec where
 
 import qualified Data.Map.Strict as M
+import Data.Either
 import System.FilePath ((</>))
 import Test.Hspec.Meta
 
+import Nirum.Constructs.Annotation hiding (null)
+import Nirum.Constructs.Annotation.Internal
 import Nirum.Constructs.Module (Module (Module))
-import Nirum.Package.Metadata (Target (compilePackage))
+import Nirum.Constructs.TypeDeclaration hiding (Text)
+import qualified Nirum.Package.Metadata as M
 import Nirum.Targets.Python
     ( Source (Source)
     , parseModulePath
@@ -19,7 +23,7 @@ spec = do
     describe "compilePackage" $ do
         it "returns a Map of file paths and their contents to generate" $ do
             let (Source pkg _) = makeDummySource $ Module [] Nothing
-                files = compilePackage pkg
+                files = M.compilePackage pkg
                 directoryStructure =
                     [ "src-py2" </> "foo" </> "__init__.py"
                     , "src-py2" </> "foo" </> "bar" </> "__init__.py"
@@ -34,7 +38,7 @@ spec = do
         it "creates an emtpy Python package directory if necessary" $ do
             let (Source pkg _) = makeDummySource' ["test"] (Module [] Nothing)
                                                   []
-                files = compilePackage pkg
+                files = M.compilePackage pkg
                 directoryStructure =
                     [ "src-py2" </> "test" </> "__init__.py"
                     , "src-py2" </> "test" </> "foo" </> "__init__.py"
@@ -51,7 +55,7 @@ spec = do
         it "generates renamed package dirs if renames are configured" $ do
             let (Source pkg _) = makeDummySource' [] (Module [] Nothing)
                                                   [(["foo"], ["quz"])]
-                files = compilePackage pkg
+                files = M.compilePackage pkg
                 directoryStructure =
                     [ "src-py2" </> "quz" </> "__init__.py"
                     , "src-py2" </> "quz" </> "bar" </> "__init__.py"
@@ -65,7 +69,7 @@ spec = do
             M.keysSet files `shouldBe` directoryStructure
             let (Source pkg' _) = makeDummySource' [] (Module [] Nothing)
                                                    [(["foo", "bar"], ["bar"])]
-                files' = compilePackage pkg'
+                files' = M.compilePackage pkg'
                 directoryStructure' =
                     [ "src-py2" </> "foo" </> "__init__.py"
                     , "src-py2" </> "bar" </> "__init__.py"
@@ -90,3 +94,43 @@ spec = do
         parseModulePath "foo..bar" `shouldBe` Nothing
         parseModulePath "foo.bar>" `shouldBe` Nothing
         parseModulePath "foo.bar-" `shouldBe` Nothing
+
+    describe "@numeric-constraints" $ do
+        it "fails if unsupported arguments are present" $ do
+            let Right annots = fromList
+                    [ Annotation
+                          "numeric-constraints"
+                          [("min", Integer 1), ("unsupported", Integer 2)]
+                    ]
+            compareErrorMessage annots
+
+        it "fails if unsupported arguments type is given" $ do
+            let Right annots = fromList
+                    [ Annotation
+                          "numeric-constraints"
+                          [("min", Integer 1), ("max", Text "2")]
+                    ]
+            compareErrorMessage annots
+
+        it "success" $ do
+            let Right annots = fromList
+                    [ Annotation
+                          "numeric-constraints"
+                          [("min", Integer 1), ("max", Integer 2)]
+                    ]
+            let Just result = getResult annots
+            isRight result `shouldBe` True
+      where
+          compareErrorMessage annots = do
+              let Just result = getResult annots
+              let Left errorMessage = result
+              errorMessage `shouldBe`
+                 "Unsupported arguments on @numeric-constraints"
+
+          getResult annots =
+              let
+                  unboxed = TypeDeclaration "foo" (UnboxedType "int32") annots
+                  (Source pkg _) = makeDummySource $ Module [unboxed] Nothing
+                  files = M.compilePackage pkg
+              in
+                  M.lookup ("src" </> "foo" </> "__init__.py") files
diff --git a/test/nirum_fixture/fixture/constraints.nrm b/test/nirum_fixture/fixture/constraints.nrm
new file mode 100644
index 0000000..35eab02
--- /dev/null
+++ b/test/nirum_fixture/fixture/constraints.nrm
@@ -0,0 +1,2 @@
+@numeric-constraints(min=1, max=12)
+unboxed month (int32);
diff --git a/test/python/constraints_test.py b/test/python/constraints_test.py
new file mode 100644
index 0000000..e86ab68
--- /dev/null
+++ b/test/python/constraints_test.py
@@ -0,0 +1,15 @@
+# -*- coding: utf-8 -*-
+from pytest import raises
+
+from fixture.constraints import Month
+
+
+def test_numeric_constraints():
+    month = Month(1)
+    assert month.value == 1
+
+    with raises(ValueError):
+        Month(0)
+
+    with raises(ValueError):
+        Month(13)
diff --git a/test/python/setup_test.py b/test/python/setup_test.py
index d89efa8..bacb269 100644
--- a/test/python/setup_test.py
+++ b/test/python/setup_test.py
@@ -22,7 +22,7 @@ def test_setup_metadata():
     assert set(pkg['Provides']) == {
         'fixture', 'fixture.foo', 'fixture.foo.bar', 'fixture.qux',
         'fixture.reserved_keyword_enum', 'fixture.reserved_keyword_union',
-        'fixture.types', 'fixture.alias',
+        'fixture.types', 'fixture.alias', 'fixture.constraints',
         'renamed', 'renamed.foo', 'renamed.foo.bar',
         'fixture.datetime',
         'fixture.name',
@@ -46,6 +46,7 @@ def test_module_entry_points():
         'fixture.reserved-keyword-enum', 'fixture.reserved-keyword-union',
         'fixture.types',
         'fixture.alias',
+        'fixture.constraints',
         'renames.test.foo', 'renames.test.foo.bar',
         'fixture.datetime',
         'fixture.name',