To control a distance between cars on the basis of both respective distances of forward and backward vehicles by setting the distance with the backward vehicle equal to or larger than the distance with the forward vehicle when the distance with the forward vehicle is a first prescribed value or less, and the distance with the backward vehicle is a second prescribed value or less.

The distances A and B between cars of forward and backward vehicles are calculated at fixed time intervals in response to the signals from a forward distance measuring means 13 and a backward distance measuring means 14. When the forward distance A is a prescribed value (a) or less in a comparator 23, and the backward distance B is a prescribed value (b) or less in a comparator 27, a signal is transmitted to an alarm driving circuit 31 to notify a driver of the access of the backward vehicle. When the backward distance B is the prescribed value (b) or less in non-driving operation, and the backward distance B is the forward distance A or more, the relative speed with the backward vehicle is regulated so as to be the relative speed with the forward vehicle or more, whereby the backward distance B is kept to be more than the forward distance A, so that a driver can concentrate on the avoidance of the rear-end collision to the forward vehicle without taking care of the backward vehicle.