PURPOSE: To achieve the motion suitable to working conditions of an automobile by automatically switching overdrive mechanism between direct drive status and overdrive one in accordance with the throttle opening and the speed of the automobile.

CONSTITUTION: A throttle signal generator 17 is provided with a working rod 18 which works in response to the operation of a throttle valve and when the displacement of this working rod 18 reaches a given valve, a contact point 19 is closed. An automobile speed signal generator 20 generates pulse signals in parallel with the automobile speed. The contact point 27 of a brake signal generator 23 is closed when a brake pedal 27 is pushed by the foot. Signals from each device 17, 20, 23 are input to an operation control device 29. This operation control device 29 generates actuator control signals which control working of the actuator.