PURPOSE: To provide the welding method which can form an oxide passive state film having corrosion resistance and discharging out-gases in an extremely small amt. in a weld zone and near this zone during a welding stage and the process device requiring an ultra-highly clean atmosphere.

CONSTITUTION: A back sealing gas consisting of an inert gas contg. 1ppm to 50ppm gaseous oxygen is passed to form the oxide passive state film essentially consisting of chromium oxide on the surface of the weld zone during the welding stage. The back sealing gas consisting of the inert gas contg. 1ppb to 5Oppm gaseous oxygen is passed to form the oxide passive film essentially consisting of the chromium oxide on the surface of the weld zone during the welding stage in the process device using welding in the construction of the device.