The present invention provides a transcoder having a MPEG decoder and a MPEG encoder, for changing a GOP structure and the bit rate of an encoded bitstream obtained as a result of an encoding process. The MPEG encoder receives a past encoding parameters generated at a past encoding process as a history information, and performs a present encoding process by using the past encoding parameters selectively so that the present encoding process is optimized. Furthermore, the encoder describe the past encoding parameters into the encoded bitstream as the history information so as to reuse the history information in advance encoding process. The picture quality of the video data does not deteriorate even if decoding and encoding processes are carried out repeatedly by the transcoder.