To provide a method for extracting the maximum gain of a servo controller by which a machine is vibrated by increasing a gain however the machine is prevented from largely vibrating or noise from being produced by quickly detecting the vibration to instantaneously stop the machine.

This servo controller to drive a servo motor is provided with a vibration detecting means to detect the vibration of a control system and an excitation means to add a simulation disturbing torque to give vibration in a certain level when a control gain is increased to a torque command. The magnitude of excitation is adjusted for excitation, and the vibration is detected by a vibration detecting means. Then, processing to increase a control gain until vibration in a fixed level is detected, and to add simulation disturbing torque is repeated, and the control gain when the vibration beyond the fixed level is detected by the vibration detecting means is defined as the maximum gain.