A power steering system used for a vehicle, in which a booster (11), a direction control valve (12) and a hydraulic pump (13) are connected to a hydraulic circuit. A pressure oil setting valve (15) enables a compensating pressure oil the pressure of which is higher than a circuit pressure to he extracted on the upstream side of the direction control valve (12). When a steering wheel (51) starts being turned, the compensating pressure oil is injected into a pair of reaction force chambers (38, 39) in the direction control valve (12) in the turning direction by presssure oil injection valves (16, 17). When the steering wheel (51) is turned back, a compensating pressure oil is injected into these two reaction force chambers (38, 39) in the turned back direction by the pressure oil injection valves (16, 17). A reaction force regulating valve (18) is provided in a reaction force regulating passage (26) which allows the two reaction force chambers (38, 39) to communicate with each other, and a throttle is regulated in accordance with a vehicle speed. This enables a response characteristics control operation improving the response characteristics with respect to a steering input to be carried out properly, and a speed sensitivity control operation varying the steering power in accordance with a vehicle speed to be carried out properly.