PURPOSE: To enable an early alerting by facilitating the catching and identifying of a target approaching at a low altitude and behind an obstacle utilizing sound wave which is less affected by the effect of reflection on the ground while able to reach even from behind an obstacle by diffraction.

CONSTITUTION: Sound waves generated from a target are picked up with plurality of sound wave receivers 15 and the sound waves thus picked up are sent to a plurality of phase shifters 16. Then, the sound generation source is searched by varying the level of operating phase of the sound wave with the phase shifters 16 while the phase shifters 16 are controlled with a controller 17. The signal thus varied here is sent to a signal analyzer 18 to recognize and identify the target with an analyzer 18. Thus, a target approaching at a low altitude or a target approaching from behind an obstacle can be caught and identified.