diff --git a/CHANGELOG.md b/CHANGELOG.md
index 11ee102fe1..a093cafdd1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,8 +5,9 @@
 
 ### New Features
 
-- The new “prevent editing” option that prevents documents from being unintentionally edited.
 - Append the nearest unique ancestor folder name to the document title if multiple documents with the same name are opened.
+- The new “prevent editing” option that prevents documents from being unintentionally edited.
+- Introduce the new AppleScript parameter `editable` to document objects.
 
 
 ### TODO
diff --git a/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/howto_readonly.html b/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/howto_readonly.html
index d7dec3471b..88c782d7fe 100644
--- a/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/howto_readonly.html
+++ b/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/howto_readonly.html
@@ -24,7 +24,10 @@ <h2>Preventing from being edited</h2>
 <p>CotEditor can temporarily prevent a document from being edited to avoid unintended data modifications while reading. This state is only valid while the document is open in CotEditor and is not saved to the file.</p>
 
 <ul>
-	<li>To prevent a document from being edited, choose Format &gt; Prevent Editing.</li>
+	<li>To prevent a document from being edited, do one of the following:<ul>
+		<li>choose Format &gt; Prevent Editing.</li>
+		<li>From an AppleScript, set the <code>editable</code> property of the <code>document</code> object to <code>false</code>.</li>
+	</ul></li>
 	
 	<li>To open a document as read-only, choose File &gt; Open, and select the “Open as read-only” checkbox in the Open dialog.</li>
 	
diff --git a/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript.html b/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript.html
index 4968f3e815..7c1d11e618 100644
--- a/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript.html
+++ b/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript.html
@@ -60,6 +60,9 @@ <h3>Document</h3>
 		<dt><code>text</code></dt>
 		<dd>The contents of the document. (text)</dd>
 
+		<dt><code>editable</code></dt>
+		<dd>Is the document editable? (boolean) <span class="added">new on CotEditor 5.1</span></dd>
+
 		<dt><del><code>length</code></del></dt>
 		<dd><del>The number of characters of the document in UTF-16. (int)</del> <span class="deprecated">deprecated on CotEditor 4.4</span></dd>
 
diff --git a/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript_changes.html b/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript_changes.html
index 38267aebcc..1517a01075 100644
--- a/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript_changes.html
+++ b/CotEditor/Resources/CotEditor.help/Contents/Resources/en.lproj/pgs/script_osascript_changes.html
@@ -17,6 +17,16 @@ <h1>Support different CotEditor versions in AppleScript</h1>
 <p>This page lists the previous specific changes on AppleScript support in CotEditor.</p>
 
 
+<article>
+<h2>Terminology change on CotEditor 5.1.0</h2>
+
+<section>
+	<h3>Cotrol the editability of documents</h3>
+	<p>A new property <code>editable</code> was added to the ‘document’ object.</p>
+</section>
+</article>
+
+
 <article>
 <h2>Terminology change on CotEditor 5.0.7</h2>
 
diff --git a/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/howto_readonly.html b/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/howto_readonly.html
index 0bd805a73f..05758af7ff 100644
--- a/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/howto_readonly.html
+++ b/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/howto_readonly.html
@@ -24,7 +24,10 @@ <h2>編集不可にする</h2>
 <p>意図しないデータの変更を避けるために、CotEditorは書類を一時的に編集できないようにすることができます。状態はCotEditorで書類を開いている間のみ有効であり、ファイルには保存されません。</p>
 
 <ul>
-	<li>書類を編集不可にするには、「フォーマット」＞「編集不可にする」と選択します。</li>
+	<li>書類を編集不可にするには、以下のいずれかの操作を行います：<ul>
+		<li>「フォーマット」＞「編集不可にする」と選択します。</li>
+		<li>AppleScriptで<code>document</code>オブジェクトの<code>editable</code>プロパティを<code>false</code>にします。</li>
+	</ul></li>
 	
 	<li>書類を読み出し専用で開くには、「ファイル」 ＞ 「開く」を選択し、「開く」ダイアログの「読み出し専用で開く」チェックボックスを有効にする。</li>
 	
diff --git a/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript.html b/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript.html
index 64e0203b53..42b45aa90c 100644
--- a/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript.html
+++ b/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript.html
@@ -60,6 +60,9 @@ <h3>Document</h3>
 		<dt><code>text</code></dt>
 		<dd>書類の内容文字列（text型）</dd>
 
+		<dt><code>editable</code></dt>
+		<dd>書類が編集可能かどうか（boolean型）<span class="added">CotEditor 5.1で追加</span></dd>
+
 		<dt><del><code>length</code></del></dt>
 		<dd><del>書類のUTF-16単位の字数（int型）</del><span class="deprecated">CotEditor 4.4で廃止</span></dd>
 
diff --git a/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript_changes.html b/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript_changes.html
index 90643b3e5f..547e7276a1 100644
--- a/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript_changes.html
+++ b/CotEditor/Resources/CotEditor.help/Contents/Resources/ja.lproj/pgs/script_osascript_changes.html
@@ -17,6 +17,16 @@ <h1>AppleScriptで異なるバージョンのCotEditorに対応する</h1>
 <p>このページでは、CotEditorのAppleScript対応における今までの仕様改訂を列挙しています。</p>
 
 
+<article>
+<h2>CotEditor 5.1.0での仕様改訂</h2>
+
+<section>
+	<h3>書類が編集可能かどうかの操作</h3>
+	<p>documentオブジェクトに新しく<code>editable</code>プロパティが追加されました。</p>
+</section>
+</article>
+
+
 <article>
 <h2>CotEditor 5.0.7での仕様改訂</h2>
 
diff --git a/CotEditor/Resources/CotEditor.sdef b/CotEditor/Resources/CotEditor.sdef
index 7098df0aef..9bf0f5e6de 100644
--- a/CotEditor/Resources/CotEditor.sdef
+++ b/CotEditor/Resources/CotEditor.sdef
@@ -52,6 +52,9 @@
             <contents name="text" code="ctxt" type="attribute run" access="rw" description="The whole text of the document.">
                 <cocoa key="scriptTextStorage"/>
             </contents>
+            <property name="editable" code="iEdt" type="boolean" access="rw" description="Is the document editable?">
+                <cocoa key="isEditable"/>
+            </property>
             <property name="coloring style" code="cclr" type="text" access="rw" description="The current syntax name.">
                 <cocoa key="coloringStyle"/>
             </property>
diff --git a/CotEditor/Sources/Document/Document.swift b/CotEditor/Sources/Document/Document.swift
index f57299e1f4..2f3a61c7bd 100644
--- a/CotEditor/Sources/Document/Document.swift
+++ b/CotEditor/Sources/Document/Document.swift
@@ -65,7 +65,7 @@ extension Document: EditorSource {
     
     // MARK: Public Properties
     
-    @ObservationIgnored @Published private(set) var isEditable = true  { didSet { self.invalidateRestorableState() } }
+    @ObservationIgnored @Published @objc var isEditable = true  { didSet { self.invalidateRestorableState() } }
     var isTransient = false  // untitled & empty document that was created automatically
     nonisolated(unsafe) var isVerticalText = false