Abstract: A cylinder containing gas has a valve and is connected to a delivery side through a filling pipe, a primary pipe, a first air-operated valve, a pressure reducing valve, a secondary pipe, and a second air-operated valve. Inert gas flows into the primary pipe through an air-operated valve. The primary pipe is connected to a vacuum generator through an air-operated valve and a pipe. Gas remaining in the primary pipe is purged as exhaust gas by automatically repeating leaving-pipe-in-pressurized-state purge for pressurizing the inside of the primary pipe by the inert gas and leaving the pipe in this state for 2 to 10 minutes and evacuating the pipe for 20 seconds. Gas remaining in the primary pipe is purged with high-efficiency, and the vacuum generator is stopped while the inside of the primary pipe is pressurized in the leaving-pipe-in-pressurized-state purge and the just-before-replacement purge.