A power steering device includes a steering mechanism, an electric motor for applying a steering assist force to the steering mechanism, and a first motor rotational position sensor for sensing as an actual axis phase a rotational position of a rotor of the electric motor. A control device is configured to: receive input of a signal of first actual axis phase outputted from the first motor rotational position sensor; receive input of a signal of value of electric current flowing through the electric motor; estimate as a control phase a phase of an induced voltage occurring in the electric motor, based on the signal of value of electric current; and determine whether or not the first motor rotational position sensor is abnormal, based on a difference between the first actual axis phase and the control phase.