The device includes a rolling bearing unit supporting a radial load and a magnetic bearing unit supporting an axial load and/or a bearing preload; an electromagnet fitted to a spindle housing so as to confront, on a non-contact basis, a flange shaped thrust plate mounted on a main shaft; a motor rotor of a motor for driving the shaft, and a motor stator opposed to the rotor, the shaft being driven by magnetic or Lorentz forces developed between the rotor and the stator; and a sensor detecting an axial force acting on the bearing unit, and a controller controlling the electromagnet. In this device, the stiffness of a composite spring formed by the bearing unit and a support system for the bearing unit is chosen to be higher than the negative stiffness of a composite spring of a motor part comprised of the electromagnet and the motor.