PURPOSE: To enable a single tool spindle to cover the speed range from low speed heavy cut work to high speed finishing work by interposing pressurized fluid between the tool spindle, radial bearings and a thrust bearing to avoid a direct contact between them.

CONSTITUTION: Fluid having pressure adjusted by a pressure regulator valve 83 in a pressurized fluid generator 79 is jetted from nozzles 101, 103, 105 through respective supply holes 87, 89, 91, 93 of radial and thrust bearings 9, 11, 29 so that a tool spindle 19 is supported with load bearing rigidity in the radial and thrust directions. Under this condition is operated a rotary drive unit 29 to rotate the tool sprindle 19 and machine a workpiece with a working tool attached to an end of the spindle 19. Since then pressurized fluid is interposed between the tool spindle 19 and the respective radial and thrust bearings 9, 11, 29 to avoid their direct contacts, exothermic reaction and power loss are reduced so that the single tool spindle 19 can cover the speed range from low speed heavy cut work to high speed finishing work.