A screw cap of plastic material has a cap disk and an outer cap rim located adjacent at a right angle to a circumference of the cap disk. The cap rim has an internal thread for closing a filling and/or removal socket of a container for transporting and storing liquid, and a central bung socket arranged retracted in the cap disk, wherein the bung socket has an internal thread for screwing in a bung plug. The bung plug has a cylindrical threaded sleeve with an external thread for screwing in a threaded bore of the bung plug of the cap, and a sealing end provided with a conical multiple seal of sealing rings which, when the bung plug is screwed in, are pressed against a conical inner sealing surface of a sealing projection located adjacent the threaded bore of the bung socket of the screw cap.