PURPOSE: To increase sensitivity against a ship even where non-magnetized and generating no noise and to improve resistance to mine-sweeping, by a method wherein a gravity sensor is used as a sensitivity sensor.

CONSTITUTION: When a mine having a gravimeter is placed in sea water, a change in gravity GS by which the gravimeter is influenced is determined according to the size of a vessel 0 being a target. Therefore, by setting and discriminating the threshold GL of an output signal from the gravimeter, explosion can be triggered when a vessel, being a target, which has size exceeding a given value, is present above the mine. Unless a substance having the same mass as that of the target vessel is brought overhead, the gravimeter does not detect a change in gravity. This means that unless a vessel having the same gravity as that of the target vessel is used, mine-sweeping action is impracticable, and resistance to mine-sweeping becomes excellent.