To prevent a creep phenomenon and a fretting phenomenon of a main spindle bearing of an NC automatic lathe.

In a pre-process of installing a main spindle 101 on a spindle stock 130, a front side bearing 110 is stored in a front side nesting housing 141, and is incorporated into a predetermined position of the main spindle 101 in a unitized state by applying a preload. A rear side bearing 120 is also stored in a rear side nesting housing 121, and is incorporated in a unitized state by applying the preload. After fixing the rear side nesting housing 121 to a predetermined position of the spindle stock 130, the front side nesting housing 141 fitted to a front main spindle hole 131 is fixed to the predetermined position of the spindle stock 130 by a fixing part 140 having its axis substantially orthogonal to the axis of the front main spindle hole 131 and substantially vertically crossing with a seat surface arranged on the outer periphery of the front side nesting housing 141. The creep phenomenon and the fretting phenomenon can be prevented by eliminating following rotation of an outer ring of a front side bearing 110.