In a rotary shear line in which sheet stock is fed by a feeder to a rotary shear for fly cutting by its rotating cutting edges, a synchronous speed and a synchronization length are calculated by an arithmetic unit which are supplied with the cutting length and the average feed rate of the sheet stock. Reference pulses of a frequency corresponding to the synchronous speed and the synchronization length are provided to a first numerical controller for controlling the feeder.

A second numerical controller for controlling the rotary shear. The first numerical controller controls the feeder to feed the sheet stock at a speed higher or lower than the synchronous speed in such a manner that when the sheet stock is fed at the synchronous speed, the length of the sheet stock short or excessive of the cutting length before its cutting is compensated for within a cutting period.

The second numerical controller controls the rotary shear to rotate its cutting edges at a speed higher or lower than the synchronous speed in such a manner that when the cutting edges are rotated at the synchronous speed, the circumferential length of the locus of each cutting edge short or excessive of the circumferential length of its 360 rotation is compensated for within the cutting period.