In a power steering system employing a power cylinder for steering assistance, hydraulic pressure is supplied through a pair of fluid lines to respective hydraulic chambers of the power cylinder. Check valves are provided to charge a hydraulic pressure in a communicating circuit intercommunicating the two fluid lines. Also provided is a solenoid-actuated directional control valve for relieving the hydraulic pressure charged in presence of a power steering system failure. An opening of the directional control valve varies depending on a solenoid attracting force and a pressure differential between the hydraulic pressure in the communicating circuit and hydraulic pressure in a reservoir. A relatively low exciting current is applied to the solenoid when the hydraulic pressure charged exceeds a predetermined pressure value. A relatively high exciting current is applied to the solenoid when the hydraulic pressure charged is less than or equal to the predetermined pressure value.