A ball bearing comprising: an outer bearing ring (1, 41, 71) having an outer raceway surface (1a, 41a) on an inner periphery of the outer bearing ring (1, 41, 71); an inner bearing ring (2, 31, 42) having an inner raceway surface (2a, 31a, 42a) on an outer periphery of the inner bearing ring (2, 31, 42); a plurality of balls disposed between the outer raceway surface (1a, 41a) and the inner raceway surface (2a, 31a, 42a); and a cage (3, 10, 20, 30, 40, 50, 60, 70) which is made of a resin, and by which the balls are arranged at equal intervals in a circumferential direction, wherein the cage (3, 10, 20, 30, 40, 50, 60, 70) has, in plural circumferential portions of the cage (3, 10, 20, 30, 40, 50, 60, 70), pockets (3a, 40a, 70a) maintaining circumferential positions of the balls, and wherein the pockets (3a, 40a, 70a) are open to a side surface of the cage (3, 10, 20, 30, 40, 50, 60, 70) on one side thereof in one axial direction, wherein at least one bearing ring of the outer bearing ring (1, 41, 71) and the inner bearing ring (2, 31, 42) includes a radially protruding engaging portion (1b, 31b, 42d, 71a) at a position of the inner or outer periphery thereof on one side of the raceway surface (1a, 31a, 42a) thereof, in the other axial direction, wherein the cage (3, 10, 20, 30, 40, 50, 60, 70) includes at least one engagement portion (3b, 10a, 20a, 30a, 40b, 50a, 60a, 70b) engageable with the engaging portion (1b, 31b, 42d, 71a) in the other axial direction, wherein a relationship between a pocket axial depth H of each of the pockets (3a, 40a, 70a) and a ball diameter d of a corresponding one of the balls is set at 0.15d ≤ H ≤ 0.65d, wherein the at least one engagement portion (3b, 10a, 20a, 30a, 40b, 50a, 60a, 70b) is disposed on one side of the pockets (3a, 40a, 70a), in the other axial direction, characterised in that the at least one engagement portion includes protrusions defining, between the protrusions and the engaging portion, spaces which narrow in axial dimension in the circumferential direction, and wherein an axial clearance is defined between the engaging portion and the protrusions. an outer bearing ring (1, 41, 71) having an outer raceway surface (1a, 41a) on an inner periphery of the outer bearing ring (1, 41, 71); an inner bearing ring (2, 31, 42) having an inner raceway surface (2a, 31a, 42a) on an outer periphery of the inner bearing ring (2, 31, 42); a plurality of balls disposed between the outer raceway surface (1a, 41a) and the inner raceway surface (2a, 31a, 42a); and a cage (3, 10, 20, 30, 40, 50, 60, 70) which is made of a resin, and by which the balls are arranged at equal intervals in a circumferential direction, wherein the cage (3, 10, 20, 30, 40, 50, 60, 70) has, in plural circumferential portions of the cage (3, 10, 20, 30, 40, 50, 60, 70), pockets (3a, 40a, 70a) maintaining circumferential positions of the balls, and wherein the pockets (3a, 40a, 70a) are open to a side surface of the cage (3, 10, 20, 30, 40, 50, 60, 70) on one side thereof in one axial direction, wherein at least one bearing ring of the outer bearing ring (1, 41, 71) and the inner bearing ring (2, 31, 42) includes a radially protruding engaging portion (1b, 31b, 42d, 71a) at a position of the inner or outer periphery thereof on one side of the raceway surface (1a, 31a, 42a) thereof, in the other axial direction, wherein the cage (3, 10, 20, 30, 40, 50, 60, 70) includes at least one engagement portion (3b, 10a, 20a, 30a, 40b, 50a, 60a, 70b) engageable with the engaging portion (1b, 31b, 42d, 71a) in the other axial direction, wherein a relationship between a pocket axial depth H of each of the pockets (3a, 40a, 70a) and a ball diameter d of a corresponding one of the balls is set at 0.15d ≤ H ≤ 0.65d, wherein the at least one engagement portion (3b, 10a, 20a, 30a, 40b, 50a, 60a, 70b) is disposed on one side of the pockets (3a, 40a, 70a), in the other axial direction, characterised in that the at least one engagement portion includes protrusions defining, between the protrusions and the engaging portion, spaces which narrow in axial dimension in the circumferential direction, and wherein an axial clearance is defined between the engaging portion and the protrusions.