Tool feed speed Fa (mm/min) is calculated for each tool feed step on the basis of a tool feed speed F (mm/rev) specified in advance and the actual rotational speed Sact of the spindle (rev/min). The tool feed speed Fa (mm/min) and the tool feed speed Fold of the previous cycle (initially, a value based on the maximum rotational speed S of the spindle as a tentative feed speed Fold) are compared, and if a change has been detected, a tool feed speed Fb (mm/min) for the subsequent tool feed step is calculated based on the relation between the tool feed speed Fc (mm/min) based on the subsequent maximum rotational speed S of the spindle (rev/min), the maximum rotational speed S of the spindle (rev/min), and the rotational speed Sact of the spindle (rev/min); and the tool feed speed undergoes pre-interpolation acceleration/deceleration control on the basis of the subsequent tool feed speed Fb (mm/min) and the tool feed speed Fa (mm/min).