PURPOSE: To prevent rear-end collision due to doze of driver by calculating a safety distance between cars while taking account of the speed of preceding car as well as the speed of his own car and actuating an awakening function stepwise depending on the distance between the cars.

CONSTITUTION: A radar unit 1 detects the distance Dm between one car 12 and a preceding car 22 while a vehicle speed sensor detects vehicle speed Vfm/s of one car and the speed Va of the preceding car is calculated from relative speed between the cars 12, 22. Idling time Tds, decision time TxS, and speed change rates α1m/S2, α2m/S2 of the cars 12, 22 are stored in a memory. Brake distance L1 of the preceding car 22, idling distance L2 of one car 12 and brake distance L3 of one car 12 are then determined according to formulas L1=Va2/2α2, L2=(Td+Tx)Vf, and L3=Vf2/2α1. Distance D at the time when L1+D=L3+L2 is satisfied is set as a limit of safety distance Ds between cars. First and second alarm regions are provided in the vicinity of the distance Ds in order to produce an alarm and to swing a seat back back and forth thus awakening a driver. Deceleration correcting means at the time of rainy weather and an erroneous function preventive means at the time of turning are also provided.