To automatically produce proper engine brake and vehicle driving power in the case of a downhill grade or an uphill grade.

An control device is provided with a road surface grade presuming section 41 for computing a road surface grade during traveling, and a target vehicle speed deciding section 47 for setting up the target vehicle speed on the car speed at the time when the road surface grade during traveling exceeds the threshold in the case where the road surface grade during traveling becomes larger than the preset threshold, and as long as the road surface grade during traveling exceeds the threshold, a change gear ratio is controlled so that the vehicle speed may be the target vehicle speed.