PURPOSE: To detect an obstacle existing in a range of difference between the inside wheels upon turning so that the rear wheel thereof may be prevented from catching it up, by comparing a signal which is directively emitted from a signal emitting device and is reflected from the obstacle, with a reference signal to detect the presence of the obstacle so that alarm is issued.

CONSTITUTION: A signal which is oscillated to a specific frequency in an oscillator 13 is emitted outboard as an ultrasonic signal having a specific frequency from a speaker which is a signal emitting device 11 which is secured in a position that is deflected to the left or right by a predetermined angle from the center line of a vehicle. A received signal from a microphone 14 when a reflected wave from an obstacle other than the vehicle wheels is received thereby, is amplified by a selective amplifier 15, and is compared with a reference level. Thereafter, the signal is delivered, as a reflected signal, to a gate circuit 18 together with a timing signal from a timing circuit 12 and a signal from a detecting device 17 for a turned angle of the front wheel, and therefore, the distance between the vehicle wheel and the obstacle is measured with the use of an echo detecting time, thereby, if the obstacle exists in a range of difference between the inside wheels upon turning, alarm is issued.