PURPOSE: To correctly identify whether a target is at rest or not, by identifying whether or not lines of underwater sound arriving directions which successively change in accordance with course control concentrate on one point.

CONSTITUTION: When approaching a target, an underwater navigating body detects the arriving direction of underwater sound from the target by using a hydrophone 1. A track calculator 2 detects the course, position, and velocity of the underwater navigating body. A course attitude controller 3, in which the outputs of the hydrophone 1 and calculator 2 are inputted, controls the course so that the course of the underwater navigating body can maintain a fixed offset angle with the arriving direction of the underwater sound from the target. Then a target discriminator 4 discriminates whether the target is at rest or not by identifying whether or not lines of the underwater sound arriving directions which successively change in accordance with course control concentrate on one point.