PURPOSE: To facilitate an easy control work, by securing a setting for both the value of necessary each constant and the position of folding point through a single control of each variable resistance.

CONSTITUTION: The clamping set value is made minimum for the clamping circuit 4, and thus the machine is started in the minimum necessary speed. In this case, the variable resistance 16 within the comparator 14 is set large enough, and then the variable resistance 8 is controlled so that the machine does not stop in front of the target point with no overshoot. Then the resistance 16 is controlled while operating the machine in the minimum speed, and the resistance 16 is controlled and set at the point where the display element 19 starts its light emission. Thus the voltage can be set at the folding point. Thereafter, the clamping set value of the circuit 4 is increased, and the machine is operated in a prescribed speed. And the variable resistance 11 is controlled so as to avoid an overshoot at the target point. In such way, a single control is possible for each variable resistance, thus realizing a setting for both the necessary each constant and the folding point. Thus an extremely simple control is secured.