A conduit P is clamped within a clamping hole 16 formed by min body 1, left and right side plates 2, 4 and top plate 3. The clamping hole is maintained by a locking mechanism 5, 5a and 11, and the conduit is deformed to have a substantially square cross section. Ultrasonic signal transmission and reception elements 13a and 13b are arranged in the main body such that an ultrasonic beam emitted from one of the elements is received by the other element after the beam is reflected by the top plate.