To preclude a plurality of fellow ultrasonic detectors from being brought into ultrasonic wave interference, and to restrain detection information from being reduced.

A plurality of fish finders 1, 2, 3 is connected to displays 4, 5, 6 by a network 100, and the fish finders 1, 2, 3 are connected respectively to a transmission pulse signal leased line 101. One of the operated fish finders is set as the master fish finder under an operation of the network of equipments for a ship. The master fish finder detects the longest period out of transmission pulse periods in all the fish finders, and outputs a transmission pulse signal based on the transmission pulse period, to the other fish finders. All the fish finders synchronized using the master transmission pulse signal, and an ultrasonic signal is transmitted into water.

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