PURPOSE: To obtain the automatic gain tuning of a servo system which can execute tuning again even if a motor is in the middle of a stop from information on the vibration of a machine system even when tuning is executed at an unstable area through the use of a standard servo motor.

CONSTITUTION: In the servo system composed of the positioning unit and the like A of PC outputting the command of operation speed and the like, a fuzzy inference part B, a servo amplifier C and the inexpensive and standard servo motor D, the amplitude of current is fetched from a current sensor C5 included in the servo amplifier C by a motor current amplitude detector B1 as the input element of a fuzzy inference module B4 and it is inferred. Thus, the automatic gain tuning of the servo system is executed.