A movable center-of-gravity tube comprising a tube body (41), a center-of-gravity body (42) accommodated inside the tube body and balls (43) held on the center-of-gravity body (42) is provided, in which the center-of-gravity body (42) has a slightly smaller diameter than the diameter of an inner surface of the tube body and comprises three or more rows of ball circulation grooves (44) on its outer surface in parallel with each other, in which the ball circulation grooves (44) accommodate the balls in a circulation manner, and each of which comprises two rows connected to each other, in such manner that the balls in one groove of the ball circulation grooves rotate and circulate in contact with the inner circumferential surface of the tube body, while the balls in another groove rotate and circulate in no contact with the inner circumferential surface of the tube body, when the center-of-gravity body moves.
