To provide an acoustic communication system that makes it possible to connect a modulation device and a sound emission device with narrow band transmission means in the system in which the modulation device and the sound emission device are separated.

A modulation device comprises: a modulation processing unit for modulating a spread code having a frequency spectrum of a middle and low sound area which is a band transmission means can transmit with the use of a data code; and a transmission output unit for outputting the modulated modulation signal to the transmission means. A sound emission device comprises: a transmission reception unit for receiving the modulation signal from the transmission means; a frequency shift unit for frequency-shifting the received modulation signal to a high sound area which is a higher frequency band including a band the transmission means cannot transmit; and a sound emission unit for emitting the frequency-shifted modulation signal.