To reduce oxygen concentration as much as possible in a head space of a container after contents are filled and sealed even in a high speed seamer.

The method for substituting inert gas in a heat space of a traveling container 10 filled with contents includes a first process for providing a hood 1 with a porous plate covering a predetermined traveling region of the traveling container 10 above the region and blowing inert gas through the porous plate 2 into the head space of the traveling container 10 to substitute the inert gas for air in the head space, a second process for blowing the inert gas inside the container 10 from a blow-out nozzle inserted between the container 10 and a container lid 11 while the lid 11 is positioned to substitute the inert gas for gas in the head space and a process for putting the lid 11 over the container 10.