PURPOSE: To prevent the deformation of rotary-spindle supporting frame due to the expansion caused by heat generated at bearings along with the rotation of said rotary spindle.

CONSTITUTION: A main spindle housing 52, the greater part of its outer surface not being restricted, is allowed to freely expand in radial directions while the resultant center-line displacement is very slight. Since the greater part of the outer surface of the housing 52 is open to the atmosphere, radiation of heat into the atmosphere is effectively performed. Furthermore, since the area which comes in contact with a supporting base 54 is reduced as much as possible, the quantity of heat which is conducted from the housing 52, through this contact surface to the supporting base 54, and from here to a frame body 2, is reduced to a great extent compared with te conventional head stock which is not provided with a clearance 70.