There is provided a filling-and-sealing device and a filling-and-sealing method capable of reducing an amount of use of a replacement gas used for replacing air in a container. The filling-and-sealing device includes a filling machine that fills a container with a content fluid; a sealing machine that seals the container transferred from the filling machine with a lid; a chamber that covers the filling machine and the sealing machine; a gassing system that replaces, in the chamber, a gas in the container with a carbon dioxide gas by introducing the carbon dioxide gas as a second replacement gas supplied from a tank; and a pre-gassing system that replaces the gas in the container with a first replacement gas that remains in the chamber containing the carbon dioxide gas before processing by the gassing system.