It is intended to provide jigs enabling a worker to hold a lock ring while resisting the elastic repulsion force urging contraction of the lock ring, inward in the radial direction, without the need for propping up a heavy lock ring.There is provided a lock ring mounting/demounting jig, the jig including two jigs in pairs, the jigs (41, 42) in pairs, constituting the jig, being joined with each other by a screw bolt (8), in which the jigs in pairs are mountable on respective ends (4a, 4b) of a lock ring (4), the jigs (41, 42) each include a main block (41B, 42B) and a clamp (41C, 42C), and the jigs (41, 42, 410, 420, 411, 421) in pairs are structured such that an interval therebetween is stretchable.