PURPOSE: To detect a waterlevel accurately, by a method wherein both an ultrasonic transmitter provided at the lower part of a water tub and an ultrasonic detector fitted at the above position perpendicular to the position of the ultrasonic transmitter at the water tub, and used, and they are composed so that the waterlevel in the water tub is detected on the basis of a propagation time when the ultrasonic wave is propagated between them.
CONSTITUTION: When water is in a water tub 2, the ultrasonic wave transmitted from an ultrasonic transmitter 3 reaches an ultrasonic detector 4, in passing through the air after the ultrasonic wave passes through the water. When the water is not in the water tub 2, the ultrasonic wave passes through only the air. For that reason, in corresponding to a waterlevel in the water tub 2, the higher the water level is, the shorter a propagation time T when the ultrasonic wave is propagated between the ultrasonic transmitter element 3 and the ultrasonic detector 4. On the basis of the relation between this waterlevel and the propagation time T, the waterlevel in the water tub 2 can be detected by measurement of the propagation time T. In this way, the waterlevel in the water tub can be accurately detected.