To convert image and sound signals so as to be transmitted between communication units adopting different coding systems and multiplexes attached information at this conversion for the transmission of the result to an opposite communication unit.

A reception information decoding section 102 decodes an image signal sent from a transmission terminal 106 and an attached information multiplexer section 104 multiplexes a moving picture signal generated from an attached moving picture generating section 103 with the decoded image signal. Then a transmission information coding section 105 encodes the multiplexed signal so as to allow a reception terminal 107 adopting a different coding system from that of the transmission terminal 106 to able to decode the coded multiplexed signal.