A ball balancer for installation to a rotator that is rotatable about a rotation axis, the ball balancer comprising: a doughnut-shaped race member having an empty space therein; and a plurality of balls arranged to be movably received in the empty space of the race member, wherein the empty space of the race member is defined by inner and outer peripheral walls in the form of a circular pipe about the rotation axis, and doughnut-shaped bottom and top walls connecting upper and lower ends of the inner and outer peripheral walls to each other, wherein an inner surface of the bottom wall is configured by an inner peripheral edge portion connected to the inner peripheral wall and a rolling face connecting the inner peripheral edge portion and the outer peripheral wall to each other, wherein the inner peripheral edge portion of the inner surface of the bottom wall is provided with a plurality of ball detention portions spaced apart from one another by a predetermined distance in a peripheral direction of the race member and a plurality of anti-separation portions respectively arranged between the plurality of ball detention portions, wherein the rolling face is inclined upward from the inner peripheral edge portion of the inner surface of the bottom wall to the outer peripheral wall to allow the balls to move to the outer peripheral wall by centrifugal force applied to the balls when a rotation speed of the rotator is greater than a second rotation speed that is less than a first rotation speed causing primary resonance, characterized in that each of the ball detention portions is configured, when the rotation speed of the rotator is less than the first rotation speed, to allow one of the balls moved from the rolling face to the ball detention portion to be detained in the ball detention portion and to allow the detained ball (BB1, BB4) in the ball detention portion to come into contact with a released one of the balls (BB2, BB5) outside of the ball detention portion so as to prevent separation of the released ball (BB2, BB5) from the rolling face, and is also configured, when the rotation speed of the rotator is greater than the first rotation speed, to allow the detained ball (BB1, BB4) in the ball detention portion to be separated to the rolling face by centrifugal force applied to the detained ball (BB1, BB4), and wherein each of the plurality of anti-separation portions is configured to come into contact with the released ball (BB3, BB6) outside of the ball detention portion when the rotation speed of the rotator is less than the first rotation speed, so as to prevent separation of the released ball (BB3, BB6) from the rolling face. a doughnut-shaped race member having an empty space therein; and a plurality of balls arranged to be movably received in the empty space of the race member, wherein the empty space of the race member is defined by inner and outer peripheral walls in the form of a circular pipe about the rotation axis, and doughnut-shaped bottom and top walls connecting upper and lower ends of the inner and outer peripheral walls to each other, wherein an inner surface of the bottom wall is configured by an inner peripheral edge portion connected to the inner peripheral wall and a rolling face connecting the inner peripheral edge portion and the outer peripheral wall to each other, wherein the inner peripheral edge portion of the inner surface of the bottom wall is provided with a plurality of ball detention portions spaced apart from one another by a predetermined distance in a peripheral direction of the race member and a plurality of anti-separation portions respectively arranged between the plurality of ball detention portions, wherein the rolling face is inclined upward from the inner peripheral edge portion of the inner surface of the bottom wall to the outer peripheral wall to allow the balls to move to the outer peripheral wall by centrifugal force applied to the balls when a rotation speed of the rotator is greater than a second rotation speed that is less than a first rotation speed causing primary resonance, characterized in that each of the ball detention portions is configured, when the rotation speed of the rotator is less than the first rotation speed, to allow one of the balls moved from the rolling face to the ball detention portion to be detained in the ball detention portion and to allow the detained ball (BB1, BB4) in the ball detention portion to come into contact with a released one of the balls (BB2, BB5) outside of the ball detention portion so as to prevent separation of the released ball (BB2, BB5) from the rolling face, and is also configured, when the rotation speed of the rotator is greater than the first rotation speed, to allow the detained ball (BB1, BB4) in the ball detention portion to be separated to the rolling face by centrifugal force applied to the detained ball (BB1, BB4), and wherein each of the plurality of anti-separation portions is configured to come into contact with the released ball (BB3, BB6) outside of the ball detention portion when the rotation speed of the rotator is less than the first rotation speed, so as to prevent separation of the released ball (BB3, BB6) from the rolling face.