PURPOSE: To enable a stable communication which is strong against a disturbance by providing an oscillator which oscillates in plural frequencies corresponding to the input of a switch in a transmission part and outputting a corresponding voice which is previously stored when an ultorasonic signal is demodulated and specified frequency of signals are detected more than specified level in a reception side.

CONSTITUTION: By operating a switch, an encoder circuit 2 is made to be started, so that an analog switch 14 is selected and driven to select the specified frequency of the oscillators 13n∼13n-1. The signal is modulated, for instance in terms of AM modulation, in a modulation circuit 3 and amplified in a power amplifier 4 in order that the unltrasonic signal is radiated into the water from an ultrasonic transducer 6 through a circulator 5. The signal is inputted in a pre-amplification part 7 through the circulator 5 and amplified in the reception part. Next, after the signal is detected in a demodulation circuit 8 it is transmitted to a tone decoder circuit 9 consisting of plural frequencies of band pass filter. And when the signal level exceeds the specified value, the demodulation circuit 8 decides that the signal is inputted and transmits a carrier detection signal to a decoder circuit 10.