The present invention provides a numerical control apparatus for a machine tool, which corrects an axial displacement of a main spindle caused due to a change in preload level and occurring according to the rotation speed of the main spindle. The numerical control apparatus (3) comprises: a numerical control section (6) for numerically controlling the movement of a spindle head (2) and for outputting a rotation command; a spindle control section (7) for controlling the rotation of a main spindle (5); a preload change control section (8) for changing the level of a preload applied to bearings (36, 37); and a correcting section (9) for outputting correction data for correction of an axial displacement of the main spindle (5) caused due to a change in the preload level and occurring according to the rotation speed of the main spindle (5) to the numerical control section (6) on the basis of the rotation command.