An acoustic positioning system using a continuous frequency-modulated (FM) ultrasonic wave transmitted from a sound source source located on the sea bottom is described in this paper. This system consists of a direction detecting system and a distance-change detecting system. The accuracy of the direction detection of the tested system was2degand the accuracy of the distance-change detection was within 0.10 m for a displacement of 50 m. Combining these two systems, it is possible to detect the change of position of the vessel sensitively. Since information from the phases of demodulated signals of the received FM ultrasonic wave is used exclusively, this system is excellent in signal-to-noise ratio (S/N) in spite of the simplicity of the electronic circuit. It was confirmed in experiments in the sea that both the direction detecting system and the distance-change detecting system worked well simultaneously, and that the combined systems would be suitable as an acoustic positioning system.