PURPOSE: To facilitate the installation of a retaining ring to the inner peripheral installation part of an installation opponent member without forming the installation opponent member to a comllicated split structure.

CONSTITUTION: A retaining ring 1 which is put to a diameter contracted state is inserted into and held on the inner peripheral surface of a cylindrical jig 50 having the inside diameter equal to or less than the inside diameter of the inner peripheral installation part 103 of an installation opponent member, and the top end of the inner cylindrical jig 50 is attached with the top end of an inner peripheral installation part 103 concentrically, and the retaining ring 1 is pushed into an annular groove 104 formed on the inner peripheral surface of the inner peripheral installation part 103 from the inner peripheral surface of the cylindrical jig 50 by a piston member 40.