PURPOSE: To remarkably save the cost of an equipment and to simplify the circuit configuration, by providing an output switching part for alternately connecting that which corresponds to plural heads, to a deciding circuit of a detecting unit, so that vehicle detecting information in plural points is obtained separately by one detecting unit.

CONSTITUTION: A detecting unit U is constituted of a transmitting circuit 1 for applying an electric signal of specified frequency to heads H1WH4, a receiving circuit 2 for receiving a reflected wave signal from the heads H1WH4 and amplifying it, a band-pass filter 3 for eliminating a noise, a detecting and rectifying circuit 4 for detecting and rectifying a signal passing through the band-pass filter, a gate circuit 5 for making an output pass through when an output from the detecting and rectifying circuit is within a prescribed gate time, an ultrasonic signal transmission and reception switching part 6 for connecting the transmitting circuit 1 and the receiving circuit 2 successively with one of the heads H1W H4, a detecting information output switching part 7 for connecting an output terminal of the gate circuit 5 successively with one of detecting information output parts O1WO4 provided in accordance with the heads H1WH4, and a control part 8.