PURPOSE: To shorten a distance between a fish detector and aquatic object possible to be detected.

CONSTITUTION: A switch circuit 9 is provided between an ultrasonic transmitter- receiver (piezoelectric transducer) 5 and a band area amplifier 6 so that the ultrasonic pulse, which is transmitted from a processor 2 by the ultrasonic transmitter-receiver 5 through a power amplifier 3 and a transmit-receive switch 4, is not input to the band area amplifier 6. Rounding of the wave-form to be caused by passing of the ultrasonic pulse in the band area amplifier 6 is thereby prevented to prevent masking of the reflected wave from the aquatic object near the ultrasonic transmitter-receiver 5 to be caused by rounding of the wave-form. An oscillation line corresponding to the ultrasonic pulse is overlapped with the return signal by a display control unit 7, and is displayed in a display 8 to display the surface of the sea.