In a torque ripple suppression control effected by a periodicity disturbance observer, it is necessary to consider plant fluctuation and fluctuation of plant characteristics caused by over time usage. Thus, improvement for robustness to an identification model error has been required. For fulfilling this requirement, there is provided a system that includes a phase correction amount calculation portion 11 that derives a phase correction amount by calculating a phase of a vector trajectory drawn by a frequency component of the periodicity disturbance, a gain correction amount calculation portion 12 that derives a gain correction amount by calculating a progress speed of the vector trajectory drawn by the frequency component of the periodicity disturbance and comparing the calculated progress speed with a threshold value and a rotation vector calculation portion 13 that derives a system identification model correction value by multiplying the phase correction amount by the gain correction amount and corrects the system identification model of a periodicity disturbance observer portion 14 based on the system identification model correction value.