PURPOSE: To prevent a serious accident in scuba-diving and so on by a technical means.

CONSTITUTION: Each driver 10 is equipped with a pinger 12. The pinger 12 transmits an ultrasonic wave in the water, and a receiver arranged alongside a marine vessel 14 receives this. Signal processing is carried out on the received ultrasonic wave, and information on the diver 10 carrying each pinger 12 is displayed on an image screen of a display unit. A name, distance, azimuth, depth and so on of the diver 10 are displayed on the image screen. The name can be obtained by imparting individual discriminating information to the transmitting ultrasonic wave, and the distance can be obtained by measuring an attenuation factor while transmitting the wave on a constant level, and the azimuth can be obtained by means of a phase difference in output of two vibrators, and the depth can be obtained by imparting depth information to the transmitting ultrasonic wave, respectively.