Disclosed is an ultrasonic wave sensor which is attached to the rear face of a bumper (4). The ultrasonic wave sensor has a construction in which a matching member (3) is adhered to an ultrasonic wave transmission face of the ultrasonic wave sensor and an opposing face of the bumper (4) and has lower acoustic impedance than the ultrasonic wave transmission face and the opposing face. Fluctuation in resonance frequency can therefore be minimised and transmission efficiency can be improved and stabilised.