Spindle device for machine tool with preload adjustment of the bearing

Information regarding various types of machining may be stored in an NC program. Therefore, by controlling preload application means on the basis of the NC program, it is possible to control an amount of preload that is applied to a rolling bearing to a preload amount that can give a spindle characteristic that meets every type of machining. Thus, highly accurate machining is possible. Then, when a preload command value falls out of range of a preload as a controllable region, the preload command value is corrected so as to fall within the range of the preload. That is, an actually controlled preload is regulated to fall within a preset range of a preload. Therefore, it is possible to ensure the stiffness of the rolling bearing or prevent heating and excessive increase in contact pressure of the rolling bearing.