PURPOSE: To make it possible to change over the characteristic of a control system in accordance with the load, by giving information to change over the characteristic of control system to a characteristic I-P or a characteristic P-I from the second CPU.

CONSTITUTION: By a sub-CPU a second command signal r to a servo motor b is set. A torque command Un is formed when a current command signal Sn outputted from a servo driver a and a feedback signal -Fn are added. A servo motor (b) is driven with this torque command Un and a speed signal (y) corresponding to the output is formed. If a changeover switch (c) is turned ON by a changeover command from a main CPU 2, then the control system will be of P-I type characteristic. If the changeover switch (c) is turned OFF, then the control system will be characterized as I-P type. The structure of a control system is thus simplified.