PURPOSE: To reduce the frequency of changing grinding wheels and prevent the stoppage of a production line to the utmost in the production line of pipes, bar steels, etc. by increasing the life of the grinding wheels to the maximum without hindering a production line on the upper course side.

CONSTITUTION: In order to determine the cutting feed speed of a long-size object, first, a longest allowable cutting time which will not hinder a production pitch is calculated from the production pitch of a production line on the upper course side and the number of simultaneously cuttable objects, and the minimum cutting efficiency is obtained from this longest allowable cutting time. Based on this, the cutting feed speed, as a rule, is determined so as to obtain the minimum cutting efficiency. Further, even though the minimum cutting efficiency is basically obtained from the longest allowable cutting time, if the obtained minimum cutting efficiency is lower than the minimum limit cutting efficiency necessary for satisfying the quality required to a workpiece, the minimum limit cutting efficiency is determined to be the target cutting efficiency.