Loads that act on all the bearings that support a main spindle are derived from a variation in the state of the main spindle and, in addition, raceway surface contact pressures of the respective bearings are analyzed, so it is possible to accurately keep track of the state of each bearing. Then, when the raceway surface contact pressure exceeds a corresponding threshold, a machining condition is changed to execute control such that the raceway surface contact pressure does not exceed the corresponding threshold, and, furthermore, when the raceway surface contact pressure exceeds the corresponding threshold even when the machining condition is changed, the main spindle is controlled to stop, so it is possible to prevent a seizure of each bearing. Thus, it is possible to improve machining accuracy by controlling the main spindle with high accuracy.