diff --git a/ChangeLog b/ChangeLog
index c303ea7b..f026c438 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2008-07-01  Tatsuhiro Tsujikawa  <tujikawa at rednoah dot com>
+
+	Updated doc
+	* src/Decoder.h
+
 2008-06-30  Tatsuhiro Tsujikawa  <tujikawa at rednoah dot com>
 
 	Updated usage text for -s option.
diff --git a/src/Decoder.h b/src/Decoder.h
index 3b5c0df0..f8cb4cc9 100644
--- a/src/Decoder.h
+++ b/src/Decoder.h
@@ -40,16 +40,20 @@
 
 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;