PURPOSE: To avoid breaking of cutter and influence to previous process by detecting the moving speed of cutter, the carrying speed of metallic material and the driving current of motor for driving the cutter everytime when performing cutting and performing feedforward control of driving current.

CONSTITUTION: Everytime when performing constant length cutting of steel pipe 1, input signals from rotary encoders 4, 29 and current transformer 92 for driving motor 25 of measuring roll 3 and die 21 are fed to speed change prevension circuit 30 to calculate the travelling speed Vd of die 21 to be lowered through frictional force produced between cylindrical rolls 28a, 28a and the travel face 22a, 22a due to cutting reaction, the carrying speed Vm of steel pipe 1 to be lowered by being restricted to die 21 through a clamp 27 and the driving current f1 of motor 25 at that time. Then the movable average of accumulated data is compared with newest data and if the difference is higher than the allowable fluctuation, corrective component of driving current of motor 25 is obtained to provide a signal to current control system 9.