A process for forming dielectric films containing at least metal atoms, silicon atoms, and oxygen atoms on a silicon substrate comprises a first step of oxidizing a surface portion of the silicon substrate to form a silicon dioxide film; a second step of forming a metal film on the silicon dioxide film in a non-oxidizing atmosphere; a third step of heating in a non-oxidizing atmosphere to diffuse the metal atoms constituting the metal film into the silicon dioxide film; and a fourth step of oxidizing the silicon dioxide film containing the diffused metal atoms to form the film containing the metal atoms, silicon atoms, and oxygen atoms.