An electric power-assist steering system that provides a steering assist control in accordance with the steering angle right after the idle stop to reduce an unpleasant steering feeling and is installed in a vehicle having an idle stop mechanism (engine controller) that automatically performs an idle stop operation in a predetermined condition. The electric power-assist steering system includes a torque sensor, a steering angle sensor for detecting a steering angle relative to a midpoint of a steering angle that is learned, an electric motor whose assist force is controlled based on the steering torque and the steering angle, and a resolver for detecting a rotational angle of the electric motor. A steering angle right after the idle stop is determined based on the steering angle detected by the steering angle sensor right before the idle stop and variation in the rotational angle detected by the resolver during the idle stop.