PURPOSE: To provide a filling method for very little quantity gas with which high-precise mixture gas can be obtained through simple operation.

CONSTITUTION: A fixed volume gas charging part 10 consisting of a pair of shut-off valves 8 and 9 is formed in a gas filling line 3 to a gas filling container 1. After the interior of the gas filling container 1 is purged in such a state as being connected to the gas filling line 3, the gas filling container 1 is brought into a reduced pressure state and communication of the gas filling container 1 with the gas filling line 3 is disconnected. After the gas filling line 3 is purged by additive gas, the additive gas is fed and charged at a specified pressure in the gas charging part 10. After, by communicating the gas charging part 10 with the gas filling container 1, the additive gas in the gas charging part 10 is transferred into the gas filling container 1 through a differential pressure, communication between the gas filling container and the gas filling line 3 is disconnected and the gas filling line 3 is purged by balance gas. Thereafter, the balance gas is fed into the gas filling container 1 to produce mixture gas of several hundreds - several ppm.