The present disclosure is to realize a fish counting device which can simply count fish at low cost. The fish counting device 1 includes an ultrasonic transducer 2 and a processor 4. The ultrasonic transducer 2 transmits an ultrasonic beam 9 underwater in a first fishtank 11 connectable with a second fishtank 12 through a path 13, and is provided with a transducer element arranged in a single channel. The transducer element is disposed at a position where the ultrasonic beam 9 does not intersect the path 13. The processor 4 outputs to the ultrasonic transducer 2 a transmission signal to drive the ultrasonic transducer 2, and acquire a number of fish passing through the ultrasonic beam 9 and moving from the path 13 to the first fishtank 11 based on an echo signal inputted from the ultrasonic transducer 2 that received a reflection wave.
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