Abstract: A stopper being adapted to be fixed in a rear end portion of a syringe is provided. The stopper is reduced in diameter at a fitting portion of a rod portion of a plunger rod so as to have a diameter smaller than an inner diameter of the rear end portion and be insertable into the rear end portion. After insertion, the stopper is increased in diameter and is fixed in the rear end portion so as not to be movable in an axial direction by fitting a first recessed portion on a periphery into a second protruding portion of an inner surface of the rear end portion. The stopper has, on an inner surface, a third protruding portion having an inner diameter smaller than an outer diameter of a gasket support flange of the rod portion in a fixed state.