PURPOSE:To enable high-efficiency operation by providing a 1st process of setting a prescribed-depth range wherein a fish school detection warning is generated within a depth range based upon the bottom of the sea and a 2nd process of sending an ultrasonic wave signal into the water. CONSTITUTION:The composite echo digital signal SCD of one-time transmission is read out of a buffer memory 44 and inputted to one input terminal of a warning decision circuit 48. Here, the warning decision circuit 48 after removing a digital signal SBD corresponding to the level of the bottom of the sea from the composite echo digital signal SCD decides whether or not an echo signal within fish school intensity level width B is present and also decides whether or not the echo signal within the fish school intensity level width B is present in the fish school warning range R set on an operation panel 30; and the circuit 48 generates an output signal when there is the echo signal corresponding to a fish school, so that a buzzer 50 sounds. Consequently, a fish school on the bottom is caught excellently and the high-efficiency operation is enabled.
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