PURPOSE: To obtain an anti-collision device with simple constitution, by providing a microwave transmitter having the horizontal directivity in the forward direction and a device receiving an equal kind of microwave from the other ship and alternately operating each transmitter and receiving device at a prescribed interval.

CONSTITUTION: Microwave transmitters 1, 1' generating a microwave signal of prescribed frequency having the directivity almost in the horizontal direction with the forward direction of an intra-ship and microwave detectors 2, 2' detecting a microwave signal of the other ship and generating an alarm signal are provided, and a power switch circuit alternately supplying power at a prescribed time interval is mounted to a circuit of each transmitter and detector. As compared with a conventional radar or the like, an anti-collision device of high reliability with simple constitution can be obtained.