To fix a rotor to a main spindle with sufficient fixing strength without giving a bad effect to the main spindle and even when the main spindle is constituted by a low heat expansion material like an invar and is rotated in a high speed, in a built-in motor type main spindle device.

In this main spindle device, the rotor 15 of a motor for driving a main sindle is directly mounted on a main spindle 1. The rotor 15 is mounted on the main spindle 1 by that the main spindle is inserted in the main spindle insertion center hole 17 formed on the rotor 15 and the rotor 15 is bonded and fixed to the main spindle 1 by an adhesive 19 interposed between the outer periphery surface of the main spindle 1 and the inner periphery surface of the main spindle insertion center hole 17.