PURPOSE: To widen the beam width of transmitting/receiving wave for short maximum detectable distance and make narrow the beam width as the maximum detectable distance increases in response to selection switching of the maximum detectable distance.

CONSTITUTION: A weighting control signal corresponding to a selected maximum detectable distance is delivered from a weighting control means (weighting control section 5) to a weighting section 2 comprising weighting circuits SA1-SAN corresponding to oscillators Z1-ZN in a transceiver 4 in order to weight the amplitude of signal outputted from a transmission signal generating means (transmission signal generating section 1). Weighted signal is delivered to each oscillator in the transceiver 4 and the beam width is determined.