An insulating film used for an interlayer insulating film of a semiconductor device and having a low dielectric constant. The insulating film comprises a carbon containing silicon oxide (SiOCH) film which has Si--CH2 bond therein. The proportion of Si--CH2 bond (1360 cm-1) to Si--CH3 bond (1270 cm-1) in the insulating film is preferably in a range from 0.03 to 0.05 measured as a peak height ratio of FTIR spectrum. The insulating film according to the present invention has higher ashing tolerance and improved adhesion to SiO2 film, when compared with the conventional SiOCH film which only has CH3 group.