Additional information is multiplexed on to an analog audio signal without deteriorating the S/N ratio of the audio signal. To accomplish this, a modulator (3) carries out spectrum diffusion of a data input (Di) supplied as additional information at a data input terminal (2). A gap insert position detection block (7) detects a gap insert position in an audio signal (Si) supplied at a signal input terminal (6). A gap inserter (8) inserts the gap at the insert position detected by the gap insert position detection block (7). A modulation signal adder (9), using as a control signal the gap inserted by the gap inserter (8), multiplexes the spectrum-diffused data on the audio signal (Si).