The purpose of the present invention is to provide an electric power steering device that improves reliability of a drive device of a three-phase synchronous motor, without increasing the cost of thedrive device. This control device for an electric power steering device controls the electric power steering device in which a three-phase synchronous motor is used as auxiliary force for steering, the control device being characterized by comprising: a rotational position estimation unit for estimating the position of a rotor of the three-phase synchronous motor on the basis of a neutral point potential or a virtual neutral point potential of the three-phase synchronous motor; and a command signal computing unit that computes a command signal of the three-phase synchronous motor on the basisof the position of the rotor which has been estimated by the rotary position estimation unit.