PURPOSE: To sense foreign matters in a liquid precisely by measuring change in the electric admittance of at least two ring-shaped ultrasonic vibrators which are in proximity installation on the inner wall of a conduit pipe.

CONSTITUTION: A fluoric resin film is formed on the surface of the conduit pipe inner wall, and at least two ring-shaped ultrasonic vibrators 2a, 2b are in proximity installation in such a way as directly contacting in liquid, and ultrasonic waves are emitted by a signal transmitter 5. Admittance measuring parts 4a, 4b sense eventual foreign matters in the liquid from change in the output voltages of differential amplifiers 6a, 6b, which are based upon change in the admittance value and change in the sonic field formed in the liquid. Thus the vibrators 2a, 2b are arranged in proximity installation to serve for sensing with changing admittance for vibration of the conduit pipe, pressure of the liquid, and change of the density, and it is not likely influenced by the density, temp., and viscosity of the liquid owing to the differential amplifiers 6a, 6b. Thus accurate sensing is actuated only for foreign matters, which ensures that sensing is made with high precision.