Abstract: An Al based wiring material layer is formed on an insulating film covering the surface of a semiconductor substrate. The wiring material layer is then selectively etched by a Cl base gas by using a resist layer as a mask, to form a wiring layer. The resist layer is ashed by using a plasma of a mixed gas of an H-and-O containing gas and a F containing gas, such as O.sub.2 /CHF.sub.3 /CH.sub.3 OH, without heating the substrate. Cl components are removed in the form of HCl, and excessive H components are removed in the form of HF to suppress the generation and adherence of H.sub.2 O, thereby improving the anticorrosion performance. The substrate is not heated so that the resist is prevented from a quality change and curing, allowing the resist to be easily removed. Another ashing process may be performed thereafter by using a F containing O.sub.2 gas such as O.sub.2 /CHF.sub.3.