A power steering system includes a hydraulic power cylinder, a hydraulic pump for supplying an oil pressure to the hydraulic power cylinder, an electric motor for driving the hydraulic pump, a first detector for detecting a steering assist force to be applied to a steering wheel, a second detector for detecting an operating condition of the electric motor, and a controller. The controller includes a control section for controlling the electric motor based on the required steering assist force detected by the first detector so as to cause the hydraulic pump to produce a required oil pressure, and a determining section for determining based on the operating condition of the electric motor and the required steering assist force detected by the first detector whether or not the power steering system is abnormal.