PURPOSE: To prolong the lifetime of a thrust bearing by providing radial bearing position adjusting nuts for adjusting the fitting condition, and also by providing thrust bearing position adjusting nuts for adjusting the abutting condition of the thrust bearing to the radial bearing pushing part.

CONSTITUTION: The adjusting structure is purposed to perform the adjustment in the fitting condition of a radial bearing 10 to the taper surface 3a on the shaft side of a main shaft 3 and the adjustment in the abutting condition of a thrust bearing 11 to the outward flange 12a of a cylinder 12 or to the radial bearing 10. The structure comprises the cylinder 12, radial bearing position adjusting nuts 15 screwed on the male screw parts 3b which have been formed on the base end part of the part where the cylinder 12 of the main shaft 3 has been fitted, and thrust bearing position adjusting nuts 16 screwed on the male screw parts 12b which have been formed on the other side of the cylinder 12. Thus, the adjustment in the fitting condition of the radial bearing 10 to the taper surface 3a on the shaft side of the main shaft 3, and the adjustment in the abutting condition of the thrust bearing 11 to the radial bearing 10 can be separately carried out.