PURPOSE: To prevent mulfunction to be caused by detection of a human body as a vehicle and by interface between sensors.

CONSTITUTION: A supersonic sensor transmits supersonic pulse signal intermittently by a supersonic oscillation element 1 consisting of a main body of an oscillation element and a horn, as well as receives reflected wave from an object during the period between a transmission and the next transmission, to detect existence of the object within a specific detection range, by circuits 2, 3, 4 and 5 which process the signal. The circuit 2, 3, 4 and 5 detect changes of the reflected wave timewise to detect the specific object. By utilizing the reflected wave timewise changes, which is caused by condition and movement of the object, the specific object is detected by detection of the timewise changes, and thereby noise, for instance, mulfunction caused by interface which may appear in a case of a parking lot where a plurality of the supersonic oscillation elements 1 are provided, and mulfunction detecting human body as a vehicle, can be well prevented.