A method for measuring the concentration of fluorine gas (F2) contained in a halogen fluoride-containing gas using an analysis apparatus having a halogen fluoride-containing gas supply source, a fluorine-containing gas supply source, a tube, a capillary, and a mass spectrometer, the method including, before measuring the concentration of fluorine gas, performing passivation treatment on the tube and the capillary using a passivation gas containing a fluorine-containing gas supplied from the fluorine-containing gas supply source.