PURPOSE: To measure distance to an object of measurement accurately by determining whether or not a detection level of an ultrasonic reflection wave so stored for each detecting circuit as to correspond to a measuring distance exceeds a set level consecutively a prescribed number of times of detection.

CONSTITUTION: An ultrasonic sensor 6 emits an ultrasonic pulse toward the lower side in a precipitation tank on the basis of an ultrasonic pulse signal of a transmission pulse generating part 11 and also it receives the ultrasonic pulse reflected inside the tank and outputs it to a received wave amplifying part 12. An A/D conversion part 13 subjects an output signal of the amplifying part 12 to A/D conversion and outputs it to a distance information memory 14. CPU 9 executes computation as to whether or not a reception level of the ultrasonic pulses received by the sensor 6 from the time of measurement N times before to the time of current measurement exceeds a set level consecutively a prescribed number of times. In the case where the reception level of the ultrasonic pulse exceeds the set level consecutively the prescribed number of times, a distance to an object wherefrom the ultrasonic pulse is reflected is determined on the basis of the memory 14.