An object of the present invention is to provide a jig that does not require an operator to hold a lock ring against the elastic repulsion force of the lock ring that attempts to contract in a radial direction when the lock ring is attached to a rim base, the jig that is safe because there is no need for the operator to support the weight of the lock ring and can maintain a state in which, when the space between the ends of the lock ring is increased, the inside diameter side of the lock ring is a perfect circle or is close to a perfect circle. Thus, a lock ring mounting/demounting jig of the present invention is formed of a pair of jigs, the pair of jigs can be attached at both ends (4a, 4b) of a lock ring (4) and on the radial outside of the lock ring (4), and the angle formed by the direction in which the space between the pair of jigs expands and contracts and the longitudinal direction of each jig is changeable.