PURPOSE: To monitor a crew's falling into water from a boat by simultaneously operating a position identifying means and a warning event sensor so as to operate a system.

CONSTITUTION: Three members of the crew are expressed three transmitters 7 to 9 and when each transmitter is soaked in water, it is immediately activated to transmit an electromagnetic signal. The sensitivity of a receiver 10 is set to be smaller than that of a receiver 11, a processing circuit is operated only when a signal from the transmitter 7 to 9 is received and the processing circuit issues a warning instruction only when an output from the receiver 10 is large. An area X in which the receiver 11 generates a larger output signal is shown by a straight line segment AB, the part of a circle from B to C, a straight line segment CD and the part of a circle D to A and a detection circuit does not issue a response signal though the transmitters 7 or 8 are wet through on the boat in the state of normal sailing. But when the signal is received from the area X, the detector automatically triggers the warning signal to discharge a life buoy. Therefore, the system monitors the crew's falling into water from the boat.