A corner determinator detects a corner appearing in a traveling direction of a vehicle and determines whether a detected corner is a blind corner based upon a current position of the vehicle and road data. A blind distance calculator calculates a distance from the determined blind corner to the vehicle based upon the current position of the vehicle and the road data. A necessary engine braking amount calculator calculates, on the basis of a calculated distance, a necessary engine braking amount necessary for decelerating a current vehicle speed to a vehicle speed at which it is possible to stop around the blind corner when a braking operation is performed with a full braking amount. A required speed change stage determinator calculates a required speed change stage based upon the determined necessary engine braking amount. A transmission controller performs a downshift to a determined required speed change stage.