diff --git a/lib/fluent/plugin/compressable.rb b/lib/fluent/plugin/compressable.rb
index 2a4824a28a..fcee4b1dbd 100644
--- a/lib/fluent/plugin/compressable.rb
+++ b/lib/fluent/plugin/compressable.rb
@@ -60,7 +60,7 @@ def string_decompress(compressed_data)
         out = ''
         loop do
           gz = Zlib::GzipReader.new(io)
-          out += gz.read
+          out << gz.read
           unused = gz.unused
           gz.finish