To adjust a control parameter without oscillating a servo control system.

Each control parameter P[p](p=0, 1,...) arranged in an order so that it can be difficult for a servo control system to oscillate are set sequentially from P[0], and fluctuation quantity σ being the fluctuation of the oscillation frequency components of a torque command Tr or speed feedback quantity ωis calculated for each control parameter, and when the fluctuation quantity σreaches a prescribed value or more, it is judged that the servo control system reaches oscillation criticality. Then, the control parameter set at that time is returned to P[p-q] (q is a natural number) or P[0].