PURPOSE: To provide a rear detection sonar system for helicopter which can define the direction and position of opponent helicopter, tank and the like approaching from behind.

CONSTITUTION: Provided in a helicopter are a multitude of surveillance sensors 3a, 3b, 3c, including in each a sonic sensor and an earth magnet sensor which are casted and set on the ground, and an on-board processor which receives the sonic signal and earth magnet signal transmitted from the surveillance sensors when the noise level of the noise source detected by these surveillance sensors exceeds a standard level, and defines the kind and position of the sound source.