PURPOSE: To eliminate a communication error due to echo noise, by comparing the output of a signal detection circuit with that of a pseudo noise detection circuit, and outputting a reception signal when the amplitude of the output of the signal detection circuit is larger than that of the output of the pseudo noise detection circuit.

CONSTITUTION: At the pseudo noise generation circuit 2 of a transmitter, a pseudo noise signal with a frequency component different from a prescribed frequency is generated while a transmission signal with the prescribed frequency outputted from a signal generation circuit 1 is interrupted, and the transmission signal and the pseudo noise signal are sent by an echo sounder transmitter 3. The output of the echo sounder transmitter 3 is inputted to the echo sounder receiver of a receiver, and the signal with the prescribed frequency is detected from the output of the echo sounder receiver 4 at the signal detection circuit 5, and also, the signal with a different frequency component is detected at the pseudo noise detection circuit 6. The output of the signal detection circuit 5 is compared with that of the pseudo noise detection circuit 6 at a comparator 7, and when the amplitude of the output of the signal detection circuit 5 is larger, the reception signal is outputted. In such a way, it is possible to eliminate the communication error due to the echo noise.