To improve dielectric strength between an n+ type polysilicon film and a plug embedded in a CH in a self-aligning contact process for opening a contact hole(CH) in the laminated films of SiOx film/n+ type polysilicon film/ SiOx film.

A sulfur-based deposit formed in plasma of mixed gas of C2 F6/O2/S2Cl2 is utilized as a side-wall protecting film 8, and anisotropic dry etching of a second interlayer insulating film 4 is performed. Thereafter, the side-wall protecting film 8 is removed by wafer heating, and the initial opening size of a resist pattern 5 is recovered. When an n+ type polysilicon film 3 and a first interlayer insulating film 2 undergo dry etching sequentially, a CH 7 having the flat side wall is obtained. When an insulating side wall 9SW is formed on the side wall of the CH 7, the processed cross section of the n+ type polysilicon film 3 can be sufficiently covered and insulated.