The present invention relates to a semiconductor device manufacturing method for forming an interlayer insulating film containing a coating insulating film having a low dielectric constant. In construction, there are provided the steps of preparing a substrate 20 on a surface of which a coating insulating film 26 is formed by coating a coating liquid containing any one selected from a group consisting of silicon-containing inorganic compound and silicon-containing organic compound, and forming a protection layer 27 for covering the coating insulating film 26 by plasmanizing a first film forming gas to react, wherein the first film forming gas consists of any one selected from a group consisting of alkoxy compound having Si—H bonds and siloxane having Si—H bonds and any one oxygen-containing gas selected from a group consisting of O2, N2O, NO2, CO, CO2, and H2O.