To eliminate the need of complicated tuning work at the spot.

A current controller is provided with a disturbance observer 18 constituted of the close loop of an observer gain 15 and a nominal plant 17. A deviation between the output signal of the nominal plant 17 and a current detection value is inputted to the observer gain 15 and the output signal is added to the output signal of a feedback controller 12 so as to generate a voltage command value to the object 56 of control. A signal obtained by reducing the output signal of the disturbance observer 18 from the voltage command value is set to be the input signal of the nominal plant 17. The transmission functions P, L and Pn of the object 56 of control, the observer gain 15 and the nominal plant 17 are designed to become PL1, PnL1 and L1 in a necessary control band. The transmission function from the output signal of the feedback controller 12 to a current detection value does not depend on P but depends on Pn.