PURPOSE: To achieve a smaller size with reduction in waves penetrating into a receiving duct from a transmitting duct by mounting a pair of elements for transmitting and receiving ultrasonic waves on a transmitting duct and a receiving duct which are arranged at such an angle that the center axes of both the ducts expand at an angle symmetrical to and the same as the radiating direction of ultrasonic waves.

CONSTITUTION: A pair of a transmitting element 2 and a receiving element 3 are provided on a transmitting duct 5 and a receiving duct 6 arranged in a shape of L with an inclined surface 4. The transmitting duct 5 and the receiving duct 6 are fixed in a case 1 at a duct space l and while at an angle θ between the center axis 10 and the center axis 11 thereof that they expand in symmetry with and in the same direction as the line (P) indicating the radiating direction of ultrasonic waves. An ultrasonic wave transmitted from the transmitting element 2 is bent to the opening of the transmitting duct 5 on the inclined surface 4 and then, radiated to an object in such a radiating distribution of ultrasonic waves as indicated by the transmitting directivity 7. A part of the radiated ultrasonic wave is reflected on a detecting member 9, penetrates into the receiving duct 6 in such a receiving sensitivity distribution of ultrasonic waves as defined by the receiving directivity 8 and reaches the receiving element 3. Thus, the ultrasonic wave can be detected.