PURPOSE: To utilize ultrasonic waves to measure the position of an objective measured point in water simply and accurately even for a deep position, by setting ultrasonic transmitter and receiver for depth measurement and an ultrasonic tramsmitter for position measurement in the same transmission point in the sea.

CONSTITUTION: Box 3 having buoy 2 is installed at transmission point O in the sea just above measured pount P, and ultrasonic sensor 4 consisting of directional ultrasonic transmitter and receiver for depth measurement and non-directional ultrasonic transmitter 5 for position measurement are stored in box 3. Meanwhile, four ultrasonic receivers 81W84 for position measurement are fitted to installation points A, B, C and D on support poles 71 and 72, which are provided on provided structure 6 and are stood in the sea, at the same interval horizontally and vertically on the same vertical face in the sea. By this constitution, the ultrasonic wave is transmitted and detected, and data related to the distance between transmission point O and measured point P and distances between transmission point O and installation points A, B, C and D are obtained in data processing equipment 11, thereby determining cubic coordinates of measured point P.