To fill a container with a liquid while the container is replaced with a gas at a higher concentration than conventional.

A controller of the gas flush filling machine 1 performs pre- replacement for opening a first exhaust valve 43 and a first gas valve 37 for making the inside of a container flush with carbon dioxide gas and main replacement for closing the first gas valve and opening a second gas valve 38 and the first exhaust valve for making the inside of the container flush with pure carbon dioxide gas. Further, when the first exhaust valve is closed and the second gas valve is opened, and when pressure is applied until a pressure in the container becomes equal to or higher than that of a reservoir tank, the second gas valve is closed and the first gas valve is opened, while a liquid valve 13 is opened to discharge the second gas in the container to the reservoir tank to have the container filled with the liquid.