A ball bearing lubricated by lubricating oil, comprising: an inner ring including an inner ring raceway groove on an outer circumferential surface thereof; an outer ring including an outer ring raceway groove on an inner circumferential surface thereof; and a plurality of balls rollably arranged between the inner ring raceway groove and the outer ring raceway groove, wherein the outer ring includes at least one radial hole that extends radially through the outer ring from an outer circumferential surface thereof to the inner circumferential surface thereof; and a counter bore is provided on the inner circumferential surface of the outer ring; characterized in that the counter bore includes an inclined portion (12b1) that gradually decreases in diameter from an axial end surface toward the outer ring raceway groove, and a straight portion (12b2) that connects the inclined portion (12b1) and the outer ring raceway groove and that is parallel to a centerline of a rotation axis of the ball bearing; and that when a minor angle formed relative to a rotation axis (s) of the ball by a straight line that connects a center of the ball and an intersection point between a centerline of the radial hole and a surface of the ball is defined as λ, an axial position of the centerline of the radial hole is set to satisfy a relationship of 0° < λ ≤ 60°, wherein the rotation axis (s) of the ball is a rotation axis when the ball rolls on the outer ring raceway groove and the inner ring raceway groove during the rotation of the ball bearing, and forms an angle of substantially 90° with respect to a line representing the contact angle α. an inner ring including an inner ring raceway groove on an outer circumferential surface thereof; an outer ring including an outer ring raceway groove on an inner circumferential surface thereof; and a plurality of balls rollably arranged between the inner ring raceway groove and the outer ring raceway groove, wherein the outer ring includes at least one radial hole that extends radially through the outer ring from an outer circumferential surface thereof to the inner circumferential surface thereof; and a counter bore is provided on the inner circumferential surface of the outer ring; characterized in that the counter bore includes an inclined portion (12b1) that gradually decreases in diameter from an axial end surface toward the outer ring raceway groove, and a straight portion (12b2) that connects the inclined portion (12b1) and the outer ring raceway groove and that is parallel to a centerline of a rotation axis of the ball bearing; and that when a minor angle formed relative to a rotation axis (s) of the ball by a straight line that connects a center of the ball and an intersection point between a centerline of the radial hole and a surface of the ball is defined as λ, an axial position of the centerline of the radial hole is set to satisfy a relationship of 0° < λ ≤ 60°, wherein the rotation axis (s) of the ball is a rotation axis when the ball rolls on the outer ring raceway groove and the inner ring raceway groove during the rotation of the ball bearing, and forms an angle of substantially 90° with respect to a line representing the contact angle α.