A solenoid valve is provided in the gear box to control the flow of power steering oil. The solenoid valve is composed of a releasing spring, a piston and a plunger. The input current is varying from 0A to 1A according to the vehicle speed and is controlled by EPS control module. When the ignition switch is turned on, current is sent to the solenoid valve to push up the plunger and accordingly the piston contacting the plunger is pushed up while overcoming the spring force. As the vehicle speed increases, the current flow going into the solenoid decreases and the piston is pushed down by the spring force. When the piston is pushed up, it closes the oil passing hole, and the power steering oil pressure is transferred into the cylinder without any interruption. But when the hole is opened as the piston is going down, some of the oil coming from the rotary valve is drained into the reservoirvia the hole inside the rotary valve.