An automatic power transmission control system includes a solenoid valve which is associated with an electric or electronic controller to receive therefrom a control signal. The solenoid valve generates a hydraulic control pressure in response to the control signal. The solenoid valve supplies the control pressure to a shift valve for switching shift valve position between a higher gear position and a lower gear position. The solenoid valve is also hydraulically communicated with a hydraulic device which performs desired operations associated with gear position shifting operation. The solenoid valve supplies the control pressure to the hydraulic device for controlling operation of the latter corresponding to the selected gear position.