PURPOSE: To minimize the attenuation of ultrasonic wave and eliminate the adverse effect of noise in an ultrasonic detecting method by introducing ultrasonic wave from an ultrasonic transmitter through a tube into an article to be detected.

CONSTITUTION: When the surface of liquid 2 in a container 1 is lowered, ultrasonic wave emitted from an ultrasonic transmitter 3 is passed through a tube 5 and received into an ultrasonic reciever 7, which converts it into an electric signal. This signal is then detected through an amplifier 8 by a detector 9 to thus supply liquid to the container 1. When the liquid is supplied to the container 1 to raise the liquid level in the container 1 to thus close the lower openings of the tubes 5, 6, the ultrasonic wave does not reach the receiver to thereby stop supplying of liquid to the container 1. Since the ultrasonic wave from the transmitter 3 is introduced through the tubes 5, 6 in this case, even if the tubes are bent, signal attenuation is reduced to thus freely select the position of the transmitter and the receiver. The tubes from the transmitter can be branched into a plurality of objects to receive the ultrasonic waves from the objects at the receivers.