PURPOSE: To improve running safety and driving feeling by providing a proper alarming time for the status of a preceding vehicle and automatically shutting off an auto-cruise system when an alarm is issued.

CONSTITUTION: A vehicle-to-vehicle distance D is found by detecting a time between the reflection of laser light emitted by an owned vehicle 12 on a preceding vehicle 22 and the return, allowing an alarm to be issued when the vehicle- to-vehicle distance D is shorter than a braking distance of the owned vehicle 12, a safety vehicle-to-vehicle distance Ds defined under an empty-run condition. The safety vehicle-to-vehicle distance is computed in consideration of not only the speed of the owned vehicle but also the speed of the preceding vehicle and an auto-cruise system is shut off when the alarm is issued.