[Issue to be addressed] A structure that prevents gouging and/or tearing caused in producing a gasket when a screw is formed and a die is released.[Means for solution] A gasket provided with a female screw hole formed as a recess from a base end toward a leading end, said female screw hole including a shaft receiving hole and a screw groove formed in said shaft receiving hole, said shaft receiving hole having a diameter gradually reduced from the base end. The gasket is also applicable for a plunger, a syringe, and a prefilled syringe, and the gasket may include a looseness prevention mechanism.
