A periodic disturbance observer determines real part ÎAn and imaginary part ÎBn of an estimated current including a periodic disturbance, from value of identification identifying a system transfer function of an nth order torque ripple frequency component from a command torque to a detected torque value, with a one-dimensional complex vector having a real part P̂An and an imaginary part P̂Bn, a cosine coefficient TAn, a sine coefficient TBn, and the real part P̂An and imaginary part P̂Bn of the system transfer function; subtracts command compensating current IAn* and IBn* obtained through pulsation extracting filter GF, respectively, from the real part ÎAn and imaginary part ÎBn of the estimated current, and thereby determines estimated periodic disturbance current real part dÎAn and imaginary part dÎBn to cancel the periodic disturbance current.