PURPOSE: To detect only a rear approaching car under no influence of a fixed building and a car running in the opposite direction, and to improve security, by transmitting an ultrasonic wave to the back and detecting its reflected wave from the rear approaching car, and discriminating that the frequency of the received signal shifts to which side of the upper or lower in comparing with the frequency of the transmitted signal by a discriminating circuit.

CONSTITUTION: An oscillator 1 and a transmitter 2 are driven to radiate the ultrasonic wave to the back. If there is a rear approaching car, the reflected wave from the approaching car is converted by a receiver 3 into an electric signal, which is amplified by an amplifier 8. An account of relative speed this received signal has a higher frequency than the transmitted signal when the both cars A and B approach each other or a lower frequency when they depart each other. The received signal amplified by the amplifier 8 is mixed with the oscillation signal from the oscillator 1 by a couple of mixers 41 and 42 and detected to obtain a couple of 90° out-of-phase signals. The deciding circuit 5 detects the presence of the approaching car, i.e. the presence of upward- shifted signal on the basis of the relative phase relation between those 90° out-of-phase signals to give a driver a warning of the presence of the approaching car by an alarm device 6.