PURPOSE: To provide a rotational balance regulating method for a machine tool spindle to make its high speed revolution possible.

CONSTITUTION: A main spindle 1 is provided with a balance regulating screw hole 1a and rotatably supported by a housing 3 through a rolling bearing 2. A stator 4 to compose a motor is fixed to the housing 3, and a rotor 5 is fixed to the main spindle 1. A disc spring 6 is attached to a draw bar 7 and inserted in the main spindle 1. A collar 8 is attached to one end of the draw bar 7, and moreover a nut 9 where a balance regulating screw hole 9a is formed is threadedly engaged with the draw bar 7. The balance of the main spindle 1 is measured with its rotation, and a weight previously regulated in its weight is inserted in the screw holes 1a, 9a to regulate the rotational balance of the main spindle 1.