To provide a calibration device for apparatus filling a gas such as hydrogen gas and capable of efficiently and safely measuring weight and amount of the gas such as hydrogen gas that are filled at high pressure. Calibration devices 100, 200, 300, 400 according to the present invention include a measurement housing 1, accommodated in a housing main body 10, to the measurement housing 1 a high pressure fuel gas being fed from outside of the housing main body 10; and a scale 3 for measuring a weight of a fuel gas fed to the measurement housing 1, wherein an inside of the measurement housing 1 is visible from outside of the housing main body 10.