An inlet conduit and an outlet conduit are disposed in a so-called crank shape with respect to a straight conduit. The straight conduit is provided with wall surfaces with which piezoelectric elements come into abutment from outside, and ultrasonic transceiver units are demountably mounted on the outside of the wall surfaces respectively. The two ultrasonic transceiver units have the same shape, and each include a covering member formed of a synthetic resin, the piezoelectric element provided in the covering member for transmitting and receiving an ultrasonic beam, and a cable connected to the piezoelectric element. The conduit portion and the ultrasonic transceiver unit are assembled separately. The ultrasonic transceiver units are disposed on both sides of the conduit portion and are coupled thereto with screws.