A method of forming a dielectric film includes: introducing a source gas essentially constituted by Si, N, H, and optionally C and having at least one bond selected from Si—N, Si—Si, and Si—H into a reaction chamber where a substrate is placed; depositing a silazane-based film essentially constituted by Si, N, H, and optionally C on the substrate by plasma reaction at −50° C. to 50° C., wherein the film is free of exposure of a solvent constituted essentially by C, H, and optionally O; and heat-treating the silazane-based film on the substrate in a heat-treating chamber while introducing an oxygen-supplying source into the heat-treating chamber to release C from the film and increase Si—O bonds in the film.