PURPOSE: To automatically remove foreign material attached to an ultrasonic vibrator.

CONSTITUTION: In an ultrasonic sensor for transmitting ultrasonic pulse signals intermittently by an ultrasonic vibrator 1 and receiving reflected waves from an obstruction between one transmission and the following transmission so as to detect the existence of the obstruction, there is appropriately provided exciting time T31 for applying the ultrasonic pulse signals continuously for the specified time longer than the time of transmission during the action of transmission so as to excite the ultrasonic vibrator 1. With this exciting time T31 provided appropriately during the action of intermittent transmission, ultrasonic vibrational energy to the extent of atomizing waterdrops attached to the vibrating face of the ultrasonic vibrator l is supplied, so that the waterdrops attached to the vibrating face of the ultrasonic vibrator 1 cannot stay thereon. This results in preventing the lowering of detecting performance caused by the attachement of waterdrops.