To enhance the balance of rotation of a main spindle during rotation at high speed.

In a main spindle device 10 in which a main spindle 12 is fitted into the cylindrical rotor 15r of a motor 15 in such a way as to be incapable of relative rotation and is supported against a housing 11 via bearings at both ends 13f, 14b thereof which project longitudinally from the rotor 15r, the main spindle 12 is longitudinally split at its portions fitting into the rotor 15r, and the spindle diameters of the main spindle 12 at both ends 13f, 14b are greater than the spindle diameters of the fitting portions 13k, 14k. Therefore, the rigidity of the main spindle 12 can be made higher than it is in conventional methods in which the spindle diameter of the main spindle at each end supported against bearings must be set as being equal to the inside diameter of the rotor.