An object detector capable of reliably detecting objects around a vehicle is mounted on a wall of a bumper of a vehicle. Thicknesses of the bumper and of a bottom surface portion of a housing are less than one-half the wavelength of ultrasonic waves generated by a piezoelectric vibrator. This suppresses the reflection of ultrasonic waves by the bumper and by the bottom surface portion of the housing, and enhances the ultrasonic wave propagation efficiency, making it possible to reliably detect objects positioned around the vehicle.