In a transmitter 2A, after an output level of a modulated signal is regulated by a first gain regulator 5A, the signal is transmitted from a first antenna 8A without delay, and after the modulated signal is delayed by a delay unit 6A and an output level of the delay output is regulated by a second gain regulator 7A, this signal is transmitted from a second antenna 9A. Similarly, in a transmitter 2B, after an output level of a modulated signal is regulated by a first gain regulator 5B, the signal is transmitted from a first antenna 8B without delay, and after the modulated signal is delayed by a delay unit 6B and an output level of the delay output is regulated by a second gain regulator 7B, this signal is transmitted from a second antenna 9B. A receiver 3 receives the transmitted signals from the four antennas 8A, 8B, 9A and 9B via an antenna 10, and executes a demodulation process.