PURPOSE: To follow up a preceding vehicle and travel the same lane by deciding the lane where the vehicle travels based on relative position and speed of the vehicle.

CONSTITUTION: A laser light emitted from a sweeping unit 12 to a measuring angle range is reflected by a preceding vehicle, and its reflected light is received by a photoreceiver 14. A distance calculator 15 measures a time interval of an emitted timing signal from a light emitting unit 11 and a receiving timing signal from the photoreceiver 14, calculates a distance to the preceding vehicle, and applies it to a signal processor 20. Thus, a relative position of the preceding vehicle is detected, and its relative speed of the vehicle is calculated. A lane where the vehicle travels is decided according to the relative position and speed. A minimum value of the distance of the vehicle decided on the same lane is output as a vehicle distance. When own speed detected by a vehicle speed sensor 16 is a predetermined set value or less, it is traveled in a follow-up traveling mode for holding a constant vehicle distance.