A method of forming a silicon oxide film, comprising the steps of: —providing a treatment substrate within a reaction chamber; —purging the gas within the reaction chamber by feeding an inert gas into the chamber under reduced pressure at a substrate temperature of 50 to 4000 C, —adsorbing, at the same temperatures and under reduced pressure, a silicon compound on the treatment substrate by pulsewise introduction of a gaseous silicon compound into the reaction chamber, —purging, at the same temperatures and under reduced pressure, the unadsorbed silicon compound in the reaction chamber with an inert gas, —at the same temperatures and under reduced pressure, introducing a pulse of ozone-containing mixed gas into the reaction chamber and producing silicon oxide by an oxidation reaction with the silicon compound adsorbed on the treatment substrate; and—repeating steps 1) to 4) if necessary to obtain the desired thickness on the substrate.