In a structure adopted for an FM-CW radar, a corner waveguide for connecting each of transmission and reception antennas to a sensor unit is integrated with a sensor unit case. In a waveguide/strip line converter for converting the transmission mode between the waveguide and the strip lines in the circuits of the sensor unit, a ditch is dug in the inner wall of an opening of the waveguide in order to prevent oozing of conductive adhesive. The strip line board is abutted on the inner wall of the waveguide in order to restrict a length of a projecting portion of a line conductor on the strip line board. By employing an oscillator mixer or a multiplier mixer, the circuitry in the sensor unit can be simplified without using lots of expensive millimeter wave devices. Furthermore, power can be utilized effectively even when a switching radar is off, which enables measurement of an absolute speed. Moreover, cost reduction can be achieved.