To provide a modulation and demodulation system capable of suppressing pitch variation of a signal due to a Doppler shift.

A modulation device 2 radiates an ultrasonic wave while approaching a modulation device 3, so a modulated signal and a pilot signal included in the ultrasonic wave (pass-band signal) picked up by a microphone shift (rise) in frequency because of the Doppler shift. Accordingly, an audio signal demodulated by a demodulator 35 has increased in pitch. At the same time, the pilot signal is folded back at a frequency fs/2 by down sampling to have transition in the opposite direction from the transition direction by the Doppler shift. Namely, the rise in frequency of the demodulated audio signal has the same value with the fall in frequency of the pilot signal by the folding-back. Consequently, a DAC 37 lowers the speed of analog conversion of the audio signal by the value of the rise in pitch of the audio signal, so that the audio signal has a proper pitch.