A bottomed, tubular hollow elastic body that opens toward an outlet side of a drug solution flow path is accommodated and arranged in an accommodation part formed in a housing, and a middle protrusion is provided protruding toward an inlet side of the drug solution flow path from a center part of a bottom wall of the hollow elastic body. Insertion of a male luer causes the bottom wall of the hollow elastic body to be elastically deformed so as to enter inside a peripheral wall, increasing the volume of the drug solution flow path, which is formed between an outer surface of the hollow elastic body and an inner surface of the accommodation part. Removing the male luer and canceling the elastic deformation of the hollow elastic body causes the volume of the drug solution flow path to be reduced.