A method for forming a deposited film by introducing a gaseous starting material for formation of a deposited film and a gaseous halogenic oxidizing agent having the property of oxidation action on said starting material separately from each other into a reaction space to form a deposited film according to a chemical reaction, which comprises activating previously a gaseous substance (B) for formation of a band gap controller in an activation space to form an activated species and introducing said activated species into the reaction space to form a deposited film controlled in band gap on a substrate existing in the film forming space.