Bearing, speed reducer, and robot

A bearing comprising:

• an inner ring located radially outside a central axis;
• an outer ring located radially outside the inner ring;
• a retainer with an annular shape located between the inner ring and the outer ring;
• and
• rollers positioned at intervals in a circumferential direction by the retainer between the inner ring and the outer ring;
• wherein
• the retainer includes:
• column portions located between adjacent pairs of the rollers;
• and
• a flange portion connecting end portions on one axial side of the column portions in a circumferential direction;
• the flange portion extends radially inside from an end portion on one axial side of the column portions;
• a radial center of the column portions is located radially outside a radial center of the rollers in a transverse plane passing through the radial center of the rollers and orthogonal to the central axis;
• and
• another axial end of the flange portion is located on another axial side with respect to one axial end of the rollers.