PURPOSE: To provide a gas filling device which is capable of filling gas mixture of refrigerant gas and inert gas into a temperature-sensitive cylinder of an expansion valve while the mixing ratio is accurately controlled, is easy to handle and does nut discharge refrigerant gas into the atmosphere.

CONSTITUTION: A refrigerant tank 11 stores liquid refrigerant in the lower part and is connected to a temperature-sensitive cylinder 1 at a position above the liquid refrigerant level. A refrigerant tank temperature controller 12 controls the temperature of the refrigerant tank 11 so that the refrigerant saturated vapor pressure agrees with the pressure of refrigerant gas to be filled in the temperature-sensitive cylinder 1. An inert gas tank 14 stores inert gas to be filled in the temperature-sensitive cylinder 1 and is connected to the refrigerant tank 11, and a pressure controller 15 controls the inner pressure of the inert gas tank 14 so that it becomes equal to the sum of the pressures of the inert gas and refrigerant gas to be filled into the temperature-sensitive cylinder 1. A check valve 18 is provided so that gas is able to flow only from the inert gas tank 14 to the refrigerant tank 11.