To improve the detection sensitivity of a vehicle position using a radio wave by improving a scattering characteristic of the surface of the radio wave reflector.

A plurality of radiation elements 13 in resonance with a radio wave are arranged in an array at an interval of (d) on the surface of the radio reflector 11. A reflected signal level is increased with respect to a resonated radio wave. The reflected signal level is selectively increased with respect to a radio wave made incident at a prescribed incident angle depending on a wavelength of a radio wave and the interval (d) so as to improve the rear scattering characteristic much more thereby improving the detection sensitivity furthermore.