PURPOSE: To send and audio signal at a proper sound pressure level without adjustment of a gain of a different source on each occasion by attenuating the audio signal based on prescribed head room information when an input audio signal is encoded.

CONSTITUTION: When an inputted audio signal S1 is coded, it is attenuated based on prescribed head room information DH1 and subjected to A/D conversion and head room information DH1 is multiplexed onto the digital audio signal D1 and the multiplexed signal is coded and the resulting audio transmission information D2 is sent. A digital audio signal D3 obtained by decoding the audio transmission information D2 is subjected to D/A conversion and its audio signal S3 is amplified based on the head room information DH2 obtained at the time of decoding to provide an output.