A Doppler radar speed sensor including an antenna section and a sensor section is mounted on the underside of the automobile undercarriage. A recess for accommodating the Doppler radar speed sensor is formed in part of the underside of the automobile undercarriage within a rectangle surrounded by the wheels, and the Doppler radar speed sensor is mounted fully within the recess. This structure prevents the sensor from being damaged in an unfavorable environment or external vehicle devices from being interfered with by the leakage of the unrequired radar wave. For size reduction, the Doppler radar speed sensor is provided with a metal casing integrated with a slotted waveguide antenna. The slot array of the antenna is formed in one side of the metal casing, so that the metal casing, makes up part of the slot array antenna.