The ultrasonic-transducer mounting structure includes: a housing including: a body part which is formed into a hollow cylindrical shape and is provided at its front surface with a transmission surface allowing an ultrasonic wave to pass therethrough and is designed to accommodate an ultrasonic transducer therein; and plural connection pieces provided to a side surface of the body part; and a holder fixed to a rear surface of a bumper and designed to hold the housing such that the transmission surface of the housing is exposed via an opening of the bumper. The holder includes plural reception pieces to which the plural connection pieces are detachably coupled respectively. The plural reception pieces are fixed to the rear surface of the bumper to surround the opening.