A control device for controlling a multi-phase AC motor generating a steering assist torque in an electric power steering device of a vehicle, includes: first and second inverters in first and second systems that output first and second alternating current voltages to first and second multi-phase winding wire sets, respectively; and a control unit that controls a phase difference between the first and second alternating current voltages. The first and second multi-phase winding wire sets provide a stator of the motor, and generate a rotating magnetic field in a rotor. The control unit sets a control range including a reference phase difference, which reduces a specified-order harmonic component, and changes the phase difference within the control range according to a characteristic required of the motor, or so as to cause a fluctuation in the energization of the motor.