PURPOSE:To prevent collision by detecting the speed of a vehicle entering into a predetermined traveling region, judging whether or not thus detected speed is within a predetermined range, and stopping the vehicle and producing an alarm if the detected speed deviates from the predetermined range. CONSTITUTION:Limit switches LS1, LS2 are disposed, while spacing apart by a predetermined distance L, at positions along a track 2 where a vehicle 3 enters from an unmanned preceding process A to line B where workers are positioned. Time (t) required by the vehicle 3 for traveling between the limit switches LS1, LS2 is then detected through a timer (not shown) and a speed V=L/t is determined. Thus determined speed V is then compared with a predetermined speed range Vo. If V<Vo, judgment is made that that wheels are abraded, the voltage is insufficient or the brake is functioning erroneously. If V>Vo, judgment is made that the control section(not shown) is operating erroneously due to noise or the source voltage is excessive. When the speed V deviates from the predetermined range Vo, an alarm is produced and the vehicle 2 is stopped and subjected to inspection or repair. By such arrangement, runaway or collision of vehicle is prevented. The invention is preferably employed in the assembly line of automobile.