Abstract: A process for the formation of a silicon oxide film, comprising the steps of exciting a gas comprising an organosilane or organosiloxane gas and a gas containing H and OH above a substrate in a reaction chamber to react them with each other in a gaseous phase or on the substrate, thereby depositing a thin film of an organic-group-containing silanol, silanol polymer, or siloxane-bonded polymer on the substrate, and removing the organic groups from the thin film to form a silicon oxide film. Preferably, the formation of a film is conducted while repeating the step of deposition and the step of removing the organic groups through a plasma treatment within an identical chamber, and the film is further heat-treated at a temperature of 450.degree. C. or below. Thus, a good insulating film having a flatness comparable to that of an SOG film can be obtained.