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.