In an electric power steering system, an ECU includes a rotation angle calculation unit, an assist torque command value calculation unit, a reference model, an angle deviation calculation unit, a PD control unit, an addition unit, and a plant observer. A proportional gain and a derivative gain which are used in the PD control unit and a first gain and a second gain which are used in a first gain multiplication unit and a second gain multiplication unit of the plant observer are set so that a characteristic equation corresponding to an equation of motion representing behavior of the electric power steering system has a double root.