PURPOSE: To enhance accuracy in main spindle rotation with the magnetic unbalance of a built-in motor maintained constant in a spindle drive device in which a spindle is driven by the built-in motor while being supported by fluid bearings.

CONSTITUTION: A main spindle 3 is driven by a built-in motor 4 while being rotatably supported by the fluid bearings 2 of a body 1. The built-in motor 4 is made up of a stator 5 at the body 1 side, and of a rotor 6 at the spindle 3 side. The stator 5 is supported on the body 1 via piezoelectric actuators 7 and springs 8, and is moved in a plane normal to the axial line of the spindle 3 by means of the piezo-electric actuators 7. The stator 5 is provided with gap sensors 9 by which each gap between the stator 5 and the rotor 6 is measured in the X and Y axes directions. A control circuit controls each piezo-electric actuator 7 based on each output of the gap sensors 9 in such a way that the magnetic center of the stator 5 interfaces with the rotating center of the rotor 6.