PURPOSE: To measure the position of a sono-buoy floating on the sea accurately, quickly, and at a low cost from an airplane by measuring the arrival time of radio waves.

CONSTITUTION: Among the sono-buoy control signals SK the address signal is sensed by a control signal sensing circuit 23, and the obtained signal is turned to a trigger signal TR, and thereupon the frequency is varied by a frequency conversion circuit 25 so that it is not duplicate with the frequency of an under- sea acoustic signal MS. The trigger signal TR is transmitted to the airplane by a multiplexer circuit 26 together with the under-sea acoustic signal MS. On the airplane side, the electric wave arriving time is measured using the sent signals, and a sono-buoy position measuring calculation circuit 18 converts the result into the corresponding distance value. This is followed by conversion into two-dimensional coordinations using different pieces of data, and the symbol mark of the airplane and the circle, whose radius represents the airplane/sono- buoy distance, are indicated on a display 19 with the aid of the coordination axes of latitude and longitude. This measuring cycle is repeated three times or more, and the latitude and longitude of the intersecting point of the circles are determined, which permits determining the position of the sono-buoy.