To provide an acoustic modulation/demodulation method which uses a sound wave as the transmission medium and enables information transmission with a simple construction.

A modulation side modulates a spread code with a data code, and the modulated spread code is differentially encoded. The differential code is multiplied by a carrier signal, is subjected to a frequency shift, and is output as a modulated signal. The differential code may be subjected to upsampling, and the modulated signal may be mixed with other acoustic signals. A demodulation side inputs the modulated signal, and performs delay detection of the modulated signal with a delay time for one tip of the differential code. Synchronization between the delay-detected signal waveform and the spread code is detected, and the data code is demodulated based on the peak polarity of the detected synchronization point.