A modulation device, a demodulation device, and a transmission system and acoustic signal transmission method using them which can carry out a synchronization process for data signals without using a carrier frequency for a synchronization signal. The modulation device transmits an acoustic signal superposed with a PN sequence set according to a carrier frequency and a data signal. A demodulation device receives the acoustic signal, determines the PN sequence having the highest maximum correlation value with the acoustic signal in a plurality of PN sequences, detects a symbol synchronization point according to the correlation value between the thus determined PN sequence and the acoustic signal, and performs a synchronization process. Thereafter, according to the detected synchronization point and determined PN sequence, an OFDM demodulation processing unit decides the carrier frequency band of the data signal superposed on the acoustic signal and demodulates the data signal.