PURPOSE: To contrive to prevent the staining of ultrasonic transmitting and receiving elements and the generation of ultrasonic interference caused by multiple reflection, by arranging ultrasonic transmitting and receiving ducts each having predetermined constitution to both sides of a circular arc shaped wavy ultrasonic buffer band at a predetermined open angle.

CONSTITUTION: L-shaped ultrasonic transmitting and receiving ducts 6, 11 each having a refractive plate 7 mounted therein are arranged to both sides of a circular arc shaped wavy ultrasonic buffer band 15 at a predetermined open angle θ so as to bring ultrasonic transmitter and receiver elements 4, 9 to the under side of the ducts 6, 11. By this constitution, the contamination of the elements 4, 9 caused by a raindrop or a stain is prevented while interference generation resulted in error detection caused by multiple reflection of ultrasonic waves is prevented by the inclination of ultrasonic transmitting and receiving wave axes corresponding to the open angle θ with respect to an object 3 to be detected and the shape of the buffer band 15.