The present invention provides a thin film-forming method by which, even when a thin film containing a crystalline metal oxide as the main component is formed over a wide area within a short time utilizing a thermal decomposition method, the thickness of the thin film becomes relatively uniform. A thin film-forming method of the present invention includes forming a thin film using a raw material containing a chloride of a metal, and prior to the forming of the thin film, 1) disposing metal-containing particles on the substrate, or 2) forming, at a film deposition rate slower than a film deposition rate for the thin film, a metal-containing thin film on the substrate, and wherein, in the case of the step 2), the thin film containing the metal oxide as the main component is directly formed on the metal-containing thin film.