PURPOSE: To stably secure the lubricant to a motion part while reducing the discharge of the lubricant and to impart no unpleasant feeling to a patient, by forming the balls of a ball bearing from ceramic and making the inner diameter of the shoulder part of at least one of the outer races of the ball bearing smaller than the diameter of the pitch circle of each of the balls.

CONSTITUTION: A supplied lubricant is ready to fly away in a radius direction by centrifugal force, because a bearing is rotated at a high speed. However, since the radius ρ1 of curvature of the track surface of an outer race has a size 60% of the diameter of each of balls, that is, said radius ρ1 of curvature is larger than that of the surface of each ball and the inner diameter Do of the shoulder part of the outer race is smaller than the diameter (d) of the pitch circle of the balls 4, 4... and the track surface 5a of the outer race 5 surrounds the outer periphery of each of the balls 4 in a size equal to or more than 180°, the lubricant is received in the recessed part of the track surface of the outer race and accumulated so as to always surround the contact part of each of the balls with the track surface. Further, the balls 4 roll in the lubricant and the film of the lubricant is given to the surface of each of the balls 4. By this method, baking resistance is enhanced coupled with the balls made of ceramic.