A cooling gas is fed from an ejection nozzle into the interior of a through-hole which is formed in a radially central portion of a main spindle, and which penetrates through said main spindle in an axial direction. The cooling gas passes through radial venting holes provided in a plurality of locations, in the circumferential direction, in an axially intermediate portion of the main spindle, is fed into axial venting holes provided in a plurality of locations, in the circumferential direction, extending from one axial-direction end of the main spindle to an intermediate portion thereof, is caused to flow through the axial venting holes, and is discharged from discharge holes.