PURPOSE: To detect a liquid by detecting an ultrasonic wave reflected by an inner construction by transmitting the ultrasonic from an outer part in detection of the liquid in the pipes provided with another construction in the inner part.

CONSTITUTION: An ultrasonic transmitter 2 is attached to an outer wall of the pipe 1 to transmit the ultrasonic against the inner part. The ultrasonic is irradiated in the diameter direction of the pipe 1, and reflected at a pipe 4 of the inner construction. An ultrasonic receiver 3 is moved slidably along the outer wall of the pipe 1, adjusted to a position where the reflected quantity is largest, i.e., the highest position of receiving sensivity, and the ultrasonic reflected at this position is received. Thereby, the presense and absence of the liquid in the inner part of the pipe 1 are detected.