A plug assembly for puncture has a cylindrical member of a rigid resin having first and second openings one at each end thereof and a channel for communication between the first and second openings. The cylindrical member has a mating tapering surface on an outer side of the first opening. The assembly also includes an elastic member of a synthetic resin arranged at least in the channel extending toward the second opening of the cylindrical member so as to block the channel of the cylindrical member. The elastic member permits a needle penetration and assuring a re-sealability after an injection needle has been withdrawn. The cylindrical member and the elastic member are molded in an integral fashion.