A hydraulic power steering system that generates a steering assist force by supplying hydraulic fluid from a hydraulic pump to a power cylinder that is coupled to a steering mechanism of a vehicle, via a hydraulic control valve that is not mechanically coupled to a steering member, characterized by comprising: steering torque detection means for detecting a steering torque (Th) that is applied to the steering member; steering angular velocity computation unit for detecting a steering angular velocity (ωh) of the steering member; a valve driving motor that is used to control an opening degree of the hydraulic control valve; opening degree command value computation means for computing an opening degree command value (θB*) for the hydraulic control valve; and means for controlling the valve driving motor based on the opening degree command value (θB*) computed by the opening degree command value computation means, wherein the opening degree command value computation means includes: basic command value setting means for setting a basic command value (TAO*) used to obtain a basic opening degree command value, using the steering torque (Th) detected by the steering torque detection means; damping control means for setting a damping control value (TD*) used to increase the steering assist force in a turning direction when a direction of the steering torque (Th) detected by the steering torque detection means is the turning direction, a magnitude of the steering torque (Th) is larger than or equal to a first threshold, a direction of the steering angular velocity (ωh) detected by the steering angular velocity computation unit is a returning direction and a magnitude of the steering angular velocity (ωh) is higher than or equal to a second threshold; and computation means for computing the opening degree command value (θB*) based on the basic command value (TAO*) set by the basic command value setting means and the damping control value (TD*) set by the damping control means. steering torque detection means for detecting a steering torque (Th) that is applied to the steering member; steering angular velocity computation unit for detecting a steering angular velocity (ωh) of the steering member; a valve driving motor that is used to control an opening degree of the hydraulic control valve; opening degree command value computation means for computing an opening degree command value (θB*) for the hydraulic control valve; and means for controlling the valve driving motor based on the opening degree command value (θB*) computed by the opening degree command value computation means, wherein the opening degree command value computation means includes: basic command value setting means for setting a basic command value (TAO*) used to obtain a basic opening degree command value, using the steering torque (Th) detected by the steering torque detection means; damping control means for setting a damping control value (TD*) used to increase the steering assist force in a turning direction when a direction of the steering torque (Th) detected by the steering torque detection means is the turning direction, a magnitude of the steering torque (Th) is larger than or equal to a first threshold, a direction of the steering angular velocity (ωh) detected by the steering angular velocity computation unit is a returning direction and a magnitude of the steering angular velocity (ωh) is higher than or equal to a second threshold; and computation means for computing the opening degree command value (θB*) based on the basic command value (TAO*) set by the basic command value setting means and the damping control value (TD*) set by the damping control means. basic command value setting means for setting a basic command value (TAO*) used to obtain a basic opening degree command value, using the steering torque (Th) detected by the steering torque detection means; damping control means for setting a damping control value (TD*) used to increase the steering assist force in a turning direction when a direction of the steering torque (Th) detected by the steering torque detection means is the turning direction, a magnitude of the steering torque (Th) is larger than or equal to a first threshold, a direction of the steering angular velocity (ωh) detected by the steering angular velocity computation unit is a returning direction and a magnitude of the steering angular velocity (ωh) is higher than or equal to a second threshold; and computation means for computing the opening degree command value (θB*) based on the basic command value (TAO*) set by the basic command value setting means and the damping control value (TD*) set by the damping control means.