diff --git a/src/Decoder.h b/src/Decoder.h
deleted file mode 100644
index e3308b80..00000000
--- a/src/Decoder.h
+++ /dev/null
@@ -1,64 +0,0 @@
-/* <!-- copyright */
-/*
- * aria2 - The high speed download utility
- *
- * Copyright (C) 2006 Tatsuhiro Tsujikawa
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program; if not, write to the Free Software
- * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
- *
- * In addition, as a special exception, the copyright holders give
- * permission to link the code of portions of this program with the
- * OpenSSL library under certain conditions as described in each
- * individual source file, and distribute linked combinations
- * including the two.
- * You must obey the GNU General Public License in all respects
- * for all of the code used other than OpenSSL.  If you modify
- * file(s) with this exception, you may extend this exception to your
- * version of the file(s), but you are not obligated to do so.  If you
- * do not wish to do so, delete this exception statement from your
- * version.  If you delete this exception statement from all source
- * files in the program, then also delete it here.
- */
-/* copyright --> */
-#ifndef D_DECODER_H
-#define D_DECODER_H
-
-#include "common.h"
-#include <string>
-
-namespace aria2 {
-
-// Interface for basic decoding functionality.
-class Decoder {
-public:
-  virtual ~Decoder() {}
-
-  // init() must be called before calling decode().
-  virtual void init() = 0;
-
-  virtual std::string decode(const unsigned char* inbuf, size_t inlen) = 0;
-
-  virtual bool finished() = 0;
-
-  // The call of release() will free allocated resources.
-  // After calling release(), the object can be reused by calling init().
-  virtual void release() = 0;
-
-  virtual const std::string& getName() const = 0;
-};
-
-} // namespace aria2
-
-#endif // D_DECODER_H
diff --git a/src/Makefile.am b/src/Makefile.am
index d129221f..186acc33 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -159,7 +159,6 @@ SRCS =  Socket.h\
 	FtpFinishDownloadCommand.cc FtpFinishDownloadCommand.h\
 	A2STR.cc A2STR.h\
 	RarestPieceSelector.cc RarestPieceSelector.h\
-	Decoder.h\
 	Signature.cc Signature.h\
 	ServerStat.cc ServerStat.h\
 	ServerStatMan.cc ServerStatMan.h\
@@ -319,8 +318,7 @@ SRCS += LibsslTLSContext.cc LibsslTLSContext.h\
 endif # HAVE_OPENSSL
 
 if HAVE_ZLIB
-SRCS += GZipDecoder.cc GZipDecoder.h\
-	GZipEncoder.cc GZipEncoder.h\
+SRCS += GZipEncoder.cc GZipEncoder.h\
 	GZipDecodingStreamFilter.cc GZipDecodingStreamFilter.h
 endif # HAVE_ZLIB
 
diff --git a/src/GZipDecoder.cc b/test/GZipDecoder.cc
similarity index 100%
rename from src/GZipDecoder.cc
rename to test/GZipDecoder.cc
diff --git a/src/GZipDecoder.h b/test/GZipDecoder.h
similarity index 87%
rename from src/GZipDecoder.h
rename to test/GZipDecoder.h
index 1ed7f29a..eacb83f2 100644
--- a/src/GZipDecoder.h
+++ b/test/GZipDecoder.h
@@ -35,35 +35,38 @@
 #ifndef D_GZIP_DECODER_H
 #define D_GZIP_DECODER_H
 
-#include "Decoder.h"
+#include "common.h"
+
+#include <string>
+
 #include <zlib.h>
 
 namespace aria2 {
 
 // GZipDecoder can decode both gzip and deflate format.
-class GZipDecoder : public Decoder {
+class GZipDecoder {
 private:
   z_stream* strm_;
 
   bool finished_;
 
   static const size_t OUTBUF_LENGTH = 16*1024;
-  
+
   static const std::string NAME;
 public:
   GZipDecoder();
 
-  virtual ~GZipDecoder();
+  ~GZipDecoder();
 
-  virtual void init();
+  void init();
 
-  virtual std::string decode(const unsigned char* inbuf, size_t inlen);
+  std::string decode(const unsigned char* inbuf, size_t inlen);
 
-  virtual bool finished();
+  bool finished();
 
-  virtual void release();
+  void release();
 
-  virtual const std::string& getName() const;
+  const std::string& getName() const;
 
 };
 
diff --git a/test/Makefile.am b/test/Makefile.am
index eb06f595..1fb52ae4 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -3,6 +3,7 @@ TESTS = aria2c
 check_PROGRAMS = $(TESTS)
 aria2c_SOURCES = AllTest.cc\
 	TestUtil.cc TestUtil.h\
+	GZipDecoder.cc GZipDecoder.h\
 	SocketCoreTest.cc\
 	array_funTest.cc\
 	Base64Test.cc\