A sinusoidal signal is superimposed on a current command value of a q-axis (torque shaft) supplied from a host, and according to the resulting current command value, the output voltage of a power converter is controlled. For the calculation of the superimposed signal added to the current command value of the q-axis, with the use of the ripple component information of induced voltage coefficients of the d-axis and q-axis of the rotating coordinate system of a permanent magnet motor, the current command values of the d-axis and q-axis, an average value of induced voltage coefficients of the d-axis, and inductance values of the d-axis and q-axis, the sinusoidal superimposed signal is calculated and added to the above-described current command value of the q-axis.