A ball bearing comprising: an inner ring ; an outer ring arranged radially outwardly of, and coaxially with, the inner ring ; a plurality of balls disposed in an annular space between the inner ring and the outer ring ; an annular seal member closing one axial end opening of the annular space ; and a cage made of resin and retaining the balls , wherein the cage comprises a cage circular annular portion extending circumferentially through a space axially sandwiched between the seal member and the space through which the balls pass; and cage claw portions having a cantilevered structure axially extending from the cage circular annular portion , and each located between a corresponding pair of the balls circumferentially adjacent to each other, characterized in that the cage circular annular portion has a cage-side sliding contact surface axially opposed to the seal member and configured to come into sliding contact with the seal member , wherein the seal member has a seal-side sliding contact surface configured to come into sliding contact with the cage-side sliding contact surface , wherein a plurality of axial protrusions each having an axially convex circular arc shape in cross sections along a circumferential direction are formed on one of the cage-side sliding contact surface and the seal-side sliding contact surface at constant pitches in the circumferential direction, and characterized in that each of the cage claw portions has, in a radially outer surface of the cage claw portion , an outer-diameter-side axial groove axially extending from a distal end of the cage claw portion toward the cage circular annular portion , and shaped such that, from the distal end of the cage claw portion toward the cage circular annular portion , a position of a bottom of the outer-diameter-side axial groove gradually changes radially outwardly. an outer ring arranged radially outwardly of, and coaxially with, the inner ring ; a plurality of balls disposed in an annular space between the inner ring and the outer ring ; an annular seal member closing one axial end opening of the annular space ; and a cage made of resin and retaining the balls , wherein the cage comprises a cage circular annular portion extending circumferentially through a space axially sandwiched between the seal member and the space through which the balls pass; and cage claw portions having a cantilevered structure axially extending from the cage circular annular portion , and each located between a corresponding pair of the balls circumferentially adjacent to each other, characterized in that the cage circular annular portion has a cage-side sliding contact surface axially opposed to the seal member and configured to come into sliding contact with the seal member , wherein the seal member has a seal-side sliding contact surface configured to come into sliding contact with the cage-side sliding contact surface , wherein a plurality of axial protrusions each having an axially convex circular arc shape in cross sections along a circumferential direction are formed on one of the cage-side sliding contact surface and the seal-side sliding contact surface at constant pitches in the circumferential direction, and characterized in that each of the cage claw portions has, in a radially outer surface of the cage claw portion , an outer-diameter-side axial groove axially extending from a distal end of the cage claw portion toward the cage circular annular portion , and shaped such that, from the distal end of the cage claw portion toward the cage circular annular portion , a position of a bottom of the outer-diameter-side axial groove gradually changes radially outwardly. <EOS>