To improve the life of a bearing device by certainly preventing the entering of foreign matter such as cutting fluid or cutting chip and dust from the outside into the inside of the bearing device.

A main spindle 1 is rotatably supported on ball bearings 2, 6 in a housing 13. A flange 12 where the main spindle 1 is pierced is attached to the end face of the housing 13. An air leading-in path 12a which is opened through the flange 12 to the annular air leading-in groove on its inner face is provided. The air leading-in path 12a is obliquely arranged along the tangential direction to the outer periphery of the main spindle 1. Thus after compressed air introduced from the air leading-in path 12a is smoothly introduced into the air leading-in groove 12b and moved around, it is uniformly jetted outward over its whole periphery from the end of the spindle 1.