PURPOSE: To prevent a rear-end collision by simple constitution such that an ultrasonic wave is emitted to detect the ultrasonic reflected wave from the obstacle on a running road and also transmitted to the rear of a car body.

CONSTITUTION: In an automatic propelling unmanned feed vehicle 1, a reflection type ultrasonic body detector 3 emits an emission wave 5 forwardly in the advance direction of said vehicle 1 and detects the reflected wave 6 thereof. When the vehicle largely inclines with respect to the preceding vehicle 2 during the running on a circumference-of-circle passage and no reflected wave 6 is detected, the presence of the preceding vehicle 2 is detected by the transmission wave 7 of the ultrasonic transmitter 4 provided to the rear part of the preceding vehicle 2 and a rear-end collision is prevented. This transmission wave 7 may be able to propagate over an extremely short distance and can be realized by simple constitution. Further, the transmission wave 7 is set to the same frequency as the emission wave 5 and subjected to intermittent emission shorter in an emission interval or continuous emission to prevent detection delay.