To detect an object by surely receiving a reflection electronic wave from an electronic wave reflector that is provided at the object.

A transmitter 10 for transmitting a linear polarization electronic wave is provided at a vehicle A, and a reflector 12 and a polarizer 14 are provided at the rear portion of a vehicle B. The polarizer 14 converts an electronic wave that is reflected from the reflector 12 into circular polarization. Even if the distance between the reflection surface of the reflector 12 and the body of the vehicle B is equal to 1/4 wavelength λ of a transmission electronic wave, a reflection electronic wave from the reflector 12 and that from the body cannot be constantly canceled since they are circular polarization and linear polarization, respectively, thus receiving the reflection electronic wave from the reflector 12.