A gasket (3) for a prefilled syringe (1) comprises a gasket body (6) and a member (7) which is adapted for mounting on the plunger. The gasket body (6) has formed on the inner surface thereof a rib (62) for retaining the member for mounting on the plunger. The member for mounting on the plunger is provided with: an outer surface protrusion section (73) engaging with the rib (62) of the gasket body (6), the rib (62) retaining the member for mounting on the plunger； elastically deformable inside protrusion sections (75) extending obliquely, from an inner section on the rear end side of the member (7) for mounting on the plunger, in the direction toward the center of the member (7)； and plunger retaining engagement sections (76) respectively formed at the free ends of the inside protrusion sections. The plunger (4) is provided with: a tubular front end section (42) capable of entering the inside of the member (7) for mounting on the plunger； and an outer surface protrusion section (43) engaging with the plunger retaining engagement sections of the member (7) for mounting on the plunger.
