A bearing in which the thickness of the bottom of at least one of the inner ring and the outer ring is 50% or more of the diameter of the balls. Further, the bearing includes an outer ring having an outer diameter of 65 mm or less and balls having a diameter of 4 mm or less and has a width dimension of 45% or less of the inner diameter dimension thereof wherein the pitch diameter of the balls is closer to the outer side of the bearing than to the center of the section of the bearing. The outer side of the outer raceway grooves is tapered, and the inner side of angularly arranged outer raceway grooves is positioned closer to the central axis of the bearing than to the outer side of the outer raceway grooves.