A ball bearing comprising: an outer raceway member having an inner periphery raceway groove; an inner raceway member having an outer periphery raceway groove; a plurality of balls disposed between the inner periphery raceway groove of the outer raceway member and the outer periphery raceway groove of the inner raceway member; and a cage that holds the balls such that the balls are arranged at intervals in a circumferential direction of the ball bearing, that is located so as to be apart from the inner raceway member, and that is allowed to be in contact with the outer raceway member, a portion of the outer raceway member, with which the cage is brought into contact, being only the inner periphery raceway groove; wherein: the cage has only one annular portion and a plurality of cage bars extending from one-side axial end portion of the annular portion toward one side in an axial direction of the ball bearing and arranged at intervals in the circumferential direction; and the cage has pockets defined between the cage bars adjacent to each other in the circumferential direction, and the balls are disposed in the pockets; and a portion of each of the cage bars, to which a corresponding one of the balls is brought into contact, is a flat face, a concave face or a convex face; characterized in that each of the cage bars of the cage is guided by the inner periphery raceway groove of the outer raceway member at two points, in a section taken along an axial direction of the ball bearing and passing through the cage bar. an outer raceway member having an inner periphery raceway groove; an inner raceway member having an outer periphery raceway groove; a plurality of balls disposed between the inner periphery raceway groove of the outer raceway member and the outer periphery raceway groove of the inner raceway member; and a cage that holds the balls such that the balls are arranged at intervals in a circumferential direction of the ball bearing, that is located so as to be apart from the inner raceway member, and that is allowed to be in contact with the outer raceway member, a portion of the outer raceway member, with which the cage is brought into contact, being only the inner periphery raceway groove; wherein: the cage has only one annular portion and a plurality of cage bars extending from one-side axial end portion of the annular portion toward one side in an axial direction of the ball bearing and arranged at intervals in the circumferential direction; and the cage has pockets defined between the cage bars adjacent to each other in the circumferential direction, and the balls are disposed in the pockets; and a portion of each of the cage bars, to which a corresponding one of the balls is brought into contact, is a flat face, a concave face or a convex face; characterized in that each of the cage bars of the cage is guided by the inner periphery raceway groove of the outer raceway member at two points, in a section taken along an axial direction of the ball bearing and passing through the cage bar. the cage has only one annular portion and a plurality of cage bars extending from one-side axial end portion of the annular portion toward one side in an axial direction of the ball bearing and arranged at intervals in the circumferential direction; and the cage has pockets defined between the cage bars adjacent to each other in the circumferential direction, and the balls are disposed in the pockets; and a portion of each of the cage bars, to which a corresponding one of the balls is brought into contact, is a flat face, a concave face or a convex face; characterized in that each of the cage bars of the cage is guided by the inner periphery raceway groove of the outer raceway member at two points, in a section taken along an axial direction of the ball bearing and passing through the cage bar.