PURPOSE: To prevent the malfunction of an ultrasonic sensor caused by another intruding object by again comparing reflection patterns when a movement detecting signal is generated after detecting the existence of an object and, in case the existence of another object is detected, extending the termination of the detecting signal by a prescribed period of time.

CONSTITUTION: When a detecting signal in a movement detecting system is generated after the existence of an intruding object, the reflection patterns from the object before and after the movement of the object is detected stored in storage circuits 17 and 18 are again compared 15. In case the reflection patterns do not coincide with each other similarly to the case where the existence of the intruding object is detected, a delay circuit 16 extends the termination of detecting output by a prescribed period of time. When a movement detecting signal is generated during the extended period of time, the detecting output is not terminated until the detecting signal in an existence detecting system obtained from the circuit 18 becomes coincident with the detecting signal stored in the circuit 17. Therefore, even when the movement of another intruding object is detected while an intruding object already exists, it does not occur that the already intruding object is misrecognized as a stationary object.