A prefilled syringe has a gasket composed of a gasket body and a plunger-mounting member mounted. The plunger-mounting member has a distal part, spiral ribs formed on an outer surface of the distal part, a flange part, and a plunger-mounting part formed on an inner surface of a hollow part. The gasket body has a spiral screwing part which screws the spiral ribs and an accommodation part for accommodating a portion where the spiral ribs are formed. A plunger has a mounting distal part. The plunger-mounting part has a spiral concave portion and an engaging rib disposed at a side distal from the spiral concave portion. The mounting distal part of the plunger has a spiral projected portion capable of screwing the spiral concave portion and an engaging portion which engages the engaging rib of the plunger-mounting member.