Abstract: A piston ring installation jig is to be used when installing a piston ring having a ring gap in a ring groove formed in an outer peripheral surface of a piston. The piston ring installation jig includes a main body including a groove. The main body has in a semi-cylindrical shape surrounding the outer peripheral surface of the piston. The groove has a substantially semicircular arc shape, and is formed in an inner peripheral surface of the main body in a region corresponding to the ring groove of the piston. A portion of the piston ring in a circumferential direction is inserted into the groove. The groove has a shape of an arc of a circle which has a circle center biased more to an outer peripheral side of the main body than a circle center of an inner peripheral surface of the main body.