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

Both of a front side bearing 110 to be a double row angular bearing and a rear side bearing 120 stored in a nesting housing 121 and imparted with pre-loads are incorporated at a prescribed position of a spindle 101 in advance before mounting the spindle 101 to a head stock 130. When the spindle 101 is mounted to the head stock 130, an outer ring of a right angular ball bearing 110R of the front side bearing 110 is fixed first in a state that an axial direction load is not applied. Subsequently, an outer ring of a left angular ball bearing 110L of the front side bearing 110 is fixed to the head stock 130, while being pressed by a thrust adjusting screw 139 screwed into a flange 137 via a wave washer 135. The corotation of the outer ring of the front side bearing 110 is eliminated to prevent the creep phenomenon and the fretting phenomenon.