A spectrum diffusion signal including data representing a center frequency of an acoustic surface filter in a transmitter side is transmitted with its carrier-frequency being swept. When a receiver side receives a modulation signal, a spectrum diffusion signal including data representing a center frequency of an acoustic surface filter in a receiver side is transmitted on the carrier-frequency equal to the center frequency of the surface acoustic wave matched filter in the transmitter side included in the modulation signal. Thereafter, the transmitter side transmits spectrum diffusion signals having the carrier-frequency equal to the center frequency of the surface acoustic wave matched filter in the receiver side included in the modulation signal.