An ultrasonic sensor module attaching device and attaching method include an ultrasonic sensor module 1 having a flange 7 of which the diameter is larger than the hole diameter of a through hole 5 prepared through a bumper 4 of a vehicle; and a fixing ring plate 3 having an engaging section 3b with the flange of the ultrasonic sensor module temporarily fixed on the back of the bumper 4 and a large-diameter fixing section 3c which extends circumferentially of the engaging section and is fixed to the back of the bumper 4; thus, the necessity for positioning the fixing ring plate 3 to the hole 5 of the bumper 4 is eliminated, which can attach easily the ultrasonic sensor module 1 to the back of the bumper.