PURPOSE: To detect the presence of an object for sure and to prevent a damage from occurring by fitting a ultrasonic generator or a receiver inside a rear bumper.

CONSTITUTION: A rear bumper 4 is constituted with an outside frame 4a and a reinforcing frame 4b, in which is arranged a ultrasonic generator 7a, which is constituted with a main body 10 enclosing a ultrasonic generating element 8 with a synthetic resin material layer 9 and a horn 11 opening toward a car rear. The born 11 has a conical thin wall 12 made in one unit with the synthetic resin material layer 9 of the main body 10, and the ultrasonic generating element 8 is arranged at the deepest section of a ultrasonic radiating aperture 13. The time duration from the time the ultrasonic wave is generated by the ultrasonic generator 7a to the time its reflected wave is received by a ultrasonic receiver 8a is measured by an electronic control unit, thus the distance from an object existing at the back of the car can be calculated based on the measured result.