To determine an existence position of a preceding vehicle in a progress direction by a method wherein transmission elements are arranged in a lattice matter, and probe waves are transmitted from each element to scan the probe waves.

A providing vehicle detector 1 comprises probe wave transmission part 2, a reflection wave reception pat 6, a control unit 10, etc. Further, the probe wave transmission part 2 comprises a plurality of transmitting circuit 3, a transmission element group 4 comprising 16 transmission elements 4A to 4P disposed in a lattice manner, and a lens 5 for converting laser beams into beam-like probe waves T. Here, the probe waves T are transmitted sequently from the transmission elements 4A to 4P, whereby the probe waves T are scanned and an existence position of a preceding vehicle is confirmed from a position of the transmission element at the time when reflection waves R are received by a reception element 8.