A gasket of a prefilled syringe is disclosed, which includes a gasket body and a plunger-mounting member. The gasket body has a plunger-mounting member removal prevention rib formed on an inner surface of the gasket body. The plunger-mounting member has an outer projected part, which engages the plunger-mounting member removal prevention rib of the gasket body, a plurality of elastically deformable inner projected parts which are extended obliquely from an inner portion of a proximal side of the plunger-mounting member toward a central and distal end of the plunger-mounting member, and plunger removal prevention locking parts formed at free ends of the inner projected parts. The plunger has a distal end part capable of entering the plunger-mounting member and an outer projection part which engages the plunger removal prevention locking parts of the plunger-mounting member.