PURPOSE: To feed a maximum effective current from a servo amplifier and to make full use of the capacity of a semiconductor by limiting current to be fed to the servo amplifier so that the estimated junction temperature of the semiconductor will not be lower than a predetermined value.

CONSTITUTION: A current detector 6 directly detects the load current (i) of a servo motor 4 which is then fed back, as a current feedback signal If, to an observer 7 and a speed control section 1. Junction temperature of a semiconductor 3 is then estimated through the observer 7 based on the load current (i) of the servo motor 4 and the duration thereof. Current to be fed from the servo motor 4 is then limited so that the junction temperature does not exceed a predetermined value at any time thus protecting servo amplifier while feeding a maximum effective current thereto.