A residual gas supply means includes a residual gas passage connected to a PSA mechanism and a heater, and a branch passage having opposite ends connected to the residual gas passage and branched from the residual gas passage. The residual gas passage has an opening-adjustable valve having an adjustable opening positioned between the connected opposite ends of the branch passage. The branch passage has a check valve, a chamber for storing the residual gas, and an on-off valve.