A controller includes a control circuit configured to calculate a target corresponding steered angle that is a target value of a corresponding steered angle based on the transmission ratio associated with the vehicle speed. The corresponding steered angle is a rotation angle of a rotation shaft, which is convertible to the steered angle whose magnitude is changed relative to a value indicating the steering angle. The control circuit is configured to control the actuation of the motor so that an actual corresponding steered angle is a final target corresponding steered angle that is based on the target corresponding steered angle. The control circuit is configured to calculate the target corresponding steered angle by adjusting the transmission ratio based on a value indicating acceleration or deceleration of a vehicle.