PURPOSE: To make it possible to perform stable servo control over a wide range of load fluctuation regardless of the causes of load fluctuation in a servo controller to servo-control a spindle motor by a driving voltage corresponding to PWM signal through pulse width modulation of error signals in feedback signals.

CONSTITUTION: This controller comprises a frequency selecting circuit 21 which has a current detecting circuit 19 for detecting a load current of a spindle motor 10, a frequency selecting circuit 21 for separating the fluctuation range of load current flowing to a spindle motor 10 into a plurality of level stages (21a, 21b and 21c) and outputting selection signals 'a' to 'd' corresponding to each level stage and a frequency setting circuit 22 for setting the frequency of PWM signals in accordance with the selection signals output in conformity with the level stage corresponding to the size judged by the frequency selecting circuit 21 from the load current detected by the current detecting circuit 19.