The present invention provides a semiconductor device manufacturing method that can sense the atmospheric air leakage more precisely and that can prevent too many defective products from being manufactured.The semiconductor device manufacturing method according to the embodiment includes the steps of: forming a barrier layer over an interlayer insulating film over a semiconductor substrate; forming a wiring layer over the barrier layer; forming a mask having an opening and configured by a photosensitive organic film over the wiring layer; patterning the wiring layer by etching the wiring layer through the opening; and removing the mask by a plasma processing using an ashing gas. The step of removing the mask includes the step of sensing an atmospheric air leakage that is mixture of the atmospheric air into the ashing gas by measuring an emission intensity of nitrogen in the ashing gas using an ultraviolet photometer.