A ball retainer, comprising: a chain belt, having a first end and a second end respectively located at two opposite end portions and a plurality of accommodation portions between the first end and the second end, wherein each of the accommodation portions is configured to accommodate a ball, characterized in that the ball retainer further comprises: a hook portion, disposed at the first end of the chain belt; and a recess portion, disposed at the second end of the chain belt corresponding to the hook portion; wherein an outer circumferential side and an inner circumferential side are formed after the hook portion of the ball retainer is snapped into the recess portion, wherein the hook portion extends toward the outer circumferential side, and the recess portion extends toward the inner circumferential side; when an acting force is applied to the chain belt during operation of the ball retainer such that the first end and the second end are moved away from each other after the hook portion is snapped into the recess portion, the hook portion is disengaged from the recess portion since the hook portion and the recess portion are weakly connected; and during operation of the ball retainer after the acting force disappears, the first end and the second end of the chain belt approach each other so that the hook portion is accordingly snapped into the recess portion. a chain belt, having a first end and a second end respectively located at two opposite end portions and a plurality of accommodation portions between the first end and the second end, wherein each of the accommodation portions is configured to accommodate a ball, characterized in that the ball retainer further comprises: a hook portion, disposed at the first end of the chain belt; and a recess portion, disposed at the second end of the chain belt corresponding to the hook portion; wherein an outer circumferential side and an inner circumferential side are formed after the hook portion of the ball retainer is snapped into the recess portion, wherein the hook portion extends toward the outer circumferential side, and the recess portion extends toward the inner circumferential side; when an acting force is applied to the chain belt during operation of the ball retainer such that the first end and the second end are moved away from each other after the hook portion is snapped into the recess portion, the hook portion is disengaged from the recess portion since the hook portion and the recess portion are weakly connected; and during operation of the ball retainer after the acting force disappears, the first end and the second end of the chain belt approach each other so that the hook portion is accordingly snapped into the recess portion. a hook portion, disposed at the first end of the chain belt; and a recess portion, disposed at the second end of the chain belt corresponding to the hook portion; wherein an outer circumferential side and an inner circumferential side are formed after the hook portion of the ball retainer is snapped into the recess portion, wherein the hook portion extends toward the outer circumferential side, and the recess portion extends toward the inner circumferential side; when an acting force is applied to the chain belt during operation of the ball retainer such that the first end and the second end are moved away from each other after the hook portion is snapped into the recess portion, the hook portion is disengaged from the recess portion since the hook portion and the recess portion are weakly connected; and during operation of the ball retainer after the acting force disappears, the first end and the second end of the chain belt approach each other so that the hook portion is accordingly snapped into the recess portion.