PROBLEM TO BE SOLVED: To solve the problem that there are many adjustment parameters, and achievement propriety of correction depends on skills of an adjuster/designer since adjustment of a gain part is determined by sequences while monitoring operating states when toque ripples are suppressed by using a periodic disturbance observer.SOLUTION: A periodic disturbance observer PDO is provided with model correction means 30 (30a). The model correction means calculates an identification model error by using time subtraction based on an output value of the periodic disturbance observer and a detected value of a plant, and calculates an identification model P'corrected by inputting the identification model error in the periodic disturbance observer. In addition, the periodic disturbance observer is provided with switch means and a memory function for learning, the identification model P'corresponding to a rotational frequency nω of the plant is stored in the memory function by on/off of the switch means, and
