Disclosed is a free ball bearing provided with: a main body comprising a ball receiving portion (23); a plurality of received balls (41) which are disposed in the ball receiving portion (23); a main ball (42) which is rotatably supported by the received balls (41); cover portions (32, 33) which surround the ball receiving portion (23) while maintaining a predetermined inner space therebetween; a received ball retaining ring (50) which is provided to be movable between a retaining position at which the received balls are retained and a releasing position at which the received balls are released; and an air hole (25a) which communicates with the inner space, wherein the received ball retaining ring (50) comprises a cylindrical portion (52) disposed between the outer peripheral surface of the ball receiving portion (23) and the inner peripheral surfaces of the cover portions (32, 33), gaps (12a, 12b) through each of which air can pass are formed between the inner peripheral surface of the cylindrical portion (52) and the outer peripheral surface of the ball receiving portion (23) and between the outer peripheral surface of the cylindrical portion (52) and the inner peripheral surfaces of the cover portions (32, 33), and both the gaps (12a, 12b) are formed to have such sizes that the force of displacement to the retaining position side is applied to the received ball retaining ring (50) when air in the inner space is sucked via the air hole (25a).