PURPOSE: To prevent any over-airing from occurring even if such a case that a user falls asleep has happened, by starting a time count automatically on the basis of a sensor operating starting command, and terminating the sensor operation when the preset operating time limit has been counted up.

CONSTITUTION: When a sensor operation starting command is inputted into a control circuit 25 by operation of a sensor operation starting switch 11 and the sensor operation is started, at each time an on-detecting signal S is inputted into the control circuit 25 of a controller 24, a motor 4 is driven at the specified driving time from that time, and if this specified time has elapsed, drive of the motor 4 is stopped. At this time, a sensor operating timer circuit 28 starts its time count automatically on the basis of the operation starting command simultaneously with a start of sensor operation, and when the count-up is over, the sensor, operation is terminated.