In a power steering device, a hydraulic power cylinder is configured to assist a steering force of a steering mechanism linked to steerable road wheels, a pump is configured to supply a hydraulic pressure to respective pressure chambers of the hydraulic power cylinder, an electric motor is configured to drive the pump, a torque sensor is configured to detect a steering torque of the steering mechanism, an operating load state detecting circuit is configured to detect an operating load state of the pump based on a viscosity resistance characteristic of a working oil, and an electric motor control circuit is configured to output a drive signal to the motor on the basis of a torque signal from the torque sensor and the operating load state of the pump.