PURPOSE: To obtain a radar detection system which is simple, small, cheap and effective even at night and in rough weather by detecting the existence of an obstacle in the blind spot of a vehicle and generating a signal showing the existence of this obstacle to the operator of the vehicle.

CONSTITUTION: A Gunn diode 1 generates a radio frequency(RF) frequency signal based on an input supplied from a timing signal to the Gunn diode 1. A single RF signal is transmitted by using a single antenna 7 and the antenna 7 effectively covers the blind spot of a specific vehicle. The RF transmission signal is reflected from the obstacle on a signal path and the antenna 7 receives a part of this reflected signal to be connected with an RF combining circuit 5 and to be furthermore connected with an RF mixer diode 9. The diode 9 generates an output with the frequency of the RF signal and a frequency equal to the difference between the frequency of the RF signal and the received reflection of the RF signal, and is connected with a signal processing section 11.