PURPOSE: To reduce the size of a built-in motor as a whole, by improving the structure for coupling the rotor of the built-in motor to its output shaft.

CONSTITUTION: The inner periphery of a clamp ring 10 is engaged with the outer periphery of the protruding end part of a sleeve part 21, which is protruded from a rotor 2. Then, an outer ring 11 is taper fit to the outer peripheral surface of the clamp ring 10, and the clamp ring 10 and the outer ring 11 are clamped together in the axial direction of the sleeve part 21 by clamp bolts 12, 12. Thereby, while contracting the diameter of the clamp ring 10, the inner periphery of the sleeve part 21 is brought into pressure contact with the outer periphery of an output shaft J.