PURPOSE: To positively suppress and control the vibration of a motor by exerting an excellent controlling capability even to such torque disturbance that excites the vibration mode of a mechanical system.

CONSTITUTION: A speed controlling method by which the speed of a motor coupled with a load through a shaft is controlled by means of a controller K based on an H∞ control theory. In order that the H∞ norms from the external signal w to the control outputs z1, z2 of a generalized plant may be under predetermined values, frequency weighting functions W1, W2, W3 are determined in such a way that, W1 whose input is the control output yz is made large in the low frequency band, W3 whose output is the input to the disturbance v is made large in the high frequence band, and W2 whose input is the control input u is made small so as to convert the oscillation mode whose damping is slow and which exists in the controlled system to an oscillation mode whose damping is quick. Thus the transfer function is determined.