PURPOSE: To make it possible to detect vessels even when they are demagnetized and turned to be anacoustic, by using a gravity sensor as a sensor constituting an underwater station.

CONSTITUTION: Gravity sensors 11W1n as underwater stations are laid at prescribed intervals on the seabed, and these gravity sensors 11W1n are connected to a land station 2 by a submerged cable 3. In the land station 2, signals from the gravity sensors 11W1n, the underwater stations, are processed for continuous monitoring. In the case when a superconducting gravity meter is used as the gravity sensor, a gravity detecting unit 10 has a vacuum vessel 12 held by a perpendicularity maintaining mechanism 11 such as a gimbal mechanism. A superconducting sphere 13 is provided in the vacuum vessel 12, and superconducting float coils 141 and 142 for floating this superconducting sphere 13 are wound in the center and the lower part of the vacuum vessel 12. In the state wherein the superconducting sphere 13 is floated, a signal from an AC bridge for position detection constituted by electrodes 15, 16 and 17 is amplified by a preamplifier circuit 22 and a tuned amplifier circuit 23 and subjected to phase detection in a phase detector circuit 24. Thereafter the signal is outputted from a feedback integration circuit 25 and sent to the land station 2 through the cable 3.