A piston-ring-installing device including a base (110) having a through-hole (111); and a guide member (120) disposed in the through-hole and fixed to the base, for positioning a piston in the vertical direction while receiving a head of the piston that is suspended downwardly in the vertical direction, and guiding a piston ring while widening a diameter of the piston ring by an outer peripheral surface of the guide member. A piston pressing member (130) is disposed above the base and supported movably toward a concave part (121b) of the guide member. The piston pressing member has a tapered inner wall surface (131) that is widened downwardly so as to come into contact with an edge of an upper end of the piston. A driving mechanism (140) is disposed on the base, and the driving mechanism is operable to drive the piston pressing member at least in the vertical direction. Also provided is a ring feeding mechanism (150) that feeds the piston ring placed on the guide member upwardly. Hence, a piston-ring installing operation is swiftly performed, and productivity is improved.