diff --git a/lib/fluent/plugin/compressable.rb b/lib/fluent/plugin/compressable.rb
index 35fc31123c..aa242478fe 100644
--- a/lib/fluent/plugin/compressable.rb
+++ b/lib/fluent/plugin/compressable.rb
@@ -64,9 +64,11 @@ def string_decompress(compressed_data)
           unused = gz.unused
           gz.finish
 
-          break if unused.nil?
-          adjust = unused.length
-          io.pos -= adjust
+          unless unused.nil?
+            adjust = unused.length
+            io.pos -= adjust
+          end
+          break if io.eof?
         end
 
         out
@@ -80,9 +82,11 @@ def io_decompress(input, output)
           unused = gz.unused
           gz.finish
 
-          break if unused.nil?
-          adjust = unused.length
-          input.pos -= adjust
+          unless unused.nil?
+            adjust = unused.length
+            input.pos -= adjust
+          end
+          break if input.eof?
         end
 
         output