A welding method for materials to be welded which are subjected to fluoride passivation treatment, and a fluoride passivation retreatment method, wherein, when fluoride passivation retreatment is conducted after welding, there is no generation of particles or dust. The method provides superior resistance to fluorine system gases. During fluoride passivation treatment, hydrogen is added to the gas (the back shield gas) flowing through the materials to be welded. In one embodiment of the welding method, the thickness of the fluoride passivated film in a predetermined range from the butt end surfaces of the materials to be welded is set to 10 nm or less, followed by subsequent welding. Furthermore, the fluoride passivation retreatment method, includes the steps of heating at least the welded parts following welding and flowing a gas containing fluorine gas in the interior portion of the parts.