To prevent the shift hunting in a downhill road for enabling shifting down or up to meet the operational feeling of occupants by estimating the grade of a road surface, and when the estimated value of the grade of the road surface in a downhill road is larger than a prescribed value, prohibiting shift-up.

Laser beams are sweepingly emitted and the time until the reflected beams of a preceding vehicle are received is measured by an inter-vehicle distance sensor 1, and the inter-vehicle distance is calculated by a distance-measuring signal processor 11. The period of the vehicle speed from a vehicle speed sensor is measured by vehicle speed signal processor 21 for detecting the speed of own vehicle. On the basis of the inter-vehicle distance and the own vehicle speed, a target inter-vehicle distance and a target own vehicle speed are calculated, and a relative speed calculating part 501 calculates the relative speed with regard to the preceding vehicle by the calculated inter-vehicle distance. Considering relative speed, an inter-vehicle distance control part 502 calculates the inter-vehicle distance so as to coincide with the target. A vehicle speed control part 51 consisting of a throttle servo part 512, a vehicle speed servo part 511, and a shift control part 513 controls the valve opening, change gear ratio, and braking force so as to obtain a target vehicle speed.