diff --git a/lib/fluent/plugin/out_exec_filter.rb b/lib/fluent/plugin/out_exec_filter.rb
index 2dec7997dd..7f62eee109 100644
--- a/lib/fluent/plugin/out_exec_filter.rb
+++ b/lib/fluent/plugin/out_exec_filter.rb
@@ -284,6 +284,7 @@ def write(chunk)
     end
 
     def run(io)
+      io.set_encoding(Encoding::ASCII_8BIT)
       case
       when @parser.implement?(:parse_io)
         @parser.parse_io(io, &method(:on_record))