PURPOSE:To display a fish school an plural indication screens by driving plural ultrasonic wave vibrators at slightly different transmission frequencies and transmitting ultrasonic waves. CONSTITUTION:Respective frequency outputs fa-fc from oscillators 401a-401c are transmitted through ultrasonic wave vibrators 2a-2c of a transmitter receiver 41, reflected by the sea bottom, etc., and received by the vibrators 2a-2c. When transmission/reception switches 41a-41c are placed at reception sides, the received signals are passed through resistors 430a-430c and added 431 together. The added received signal is amplified by a preamplifier 432 and then supplied to mixers 433a-433c. The mixers 433a-433c mix the received signals of frequencies fa-fc with outputs of frequencies fca-fcc supplied thereto an only specific frequencies among mixture outputs are passed through filters 434a-434c constituting a frequency demultiplexing means and main amplifiers 435a-435c and amplified by video amplifiers 440a-440c respectively. Their amplification outputs are swept by sweepers 441a-441c and displayed on plural CRTs 442a-442c.
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