To fill a gas-filled container with gases while the density and pressure of a mixed gas are adjusted to desired values in a process of collecting a specific gas without discharging the gas to the atmosphere even if the specific gas can not be separated completely when the mixed gas in the gas-filled container 130 is separated by a gas separation part 105 and collected, mixing the specific gas and a dilute gas which are collected temporarily, and filling the gas-filled container with them.

A gas supply part 101 which supplies the respective gases, the gas separation part 105 which separates those gases, and the gas-filled container 130 are connected by pipes; and the gases separated by the gas separation part are properly selected and connected to the gas-filled container by pipes to enable the separated gases to be put back into the gas-filled container and while the gases are circulated among those devices to measure the pressure and gas density in the gas-filled container, the return amounts of those gases are controlled to adjust the pressure and gas density in the gas-filled container to the desired values.