PURPOSE: To restrain frequent shift-down and shift-up motion and accordingly stabilize behavior of a car body as well as preventing torque shock by a shift- down line of an automatic speed change gear shift map to the non-shift-down direction at the time of constant speed travel control.

CONSTITUTION: A control unit 8 sets target throttle opening on the basis of an actual car speed and others and drives a throttle valve actuator 4 by an automatic speed control means 8A. Additionally, the control unit 8 controls an automatic speed change gear 5 by a speed change control means 8B on the basis of the actual car speed, throttle opening and others. Furthermore, at the time of a car entering an uphill road, it judges shift-down of the automatic speed change gear 5 by a 4→3 judgment means 8C. Subsequently, at the time of the car emerging from the uphill road, it judges shift-up of the automatic speed change gear 5 by a 3→4 judgment means 8D. Then, when the actual car speed goes lower than the target speed by a specified value, the car speed is improved by carrying out shift-down and converged to the target car speed. At this time, a shift-down line of a shift map of the automatic speed change gear 5 is changed to the non-shift-down direction.