A modular device for implanting radioactive seeds in an animal body through needles implanted in the body. The bag body includes a first layer and a second layer overlapped therewith, and defines a plurality of sealing pockets discretely provided between the first and second layers and a plurality of elongated sealing openings communicating with said sealing pockets respectively. These are brought into contact with a carrier gas, and a phosphorus pentafluoride is extracted and separated into the carrier gas.