The present invention provides a method of forming a thin film containing a metal oxide as the main component, the film thickness of which is relatively uniform, at a high film deposition rate over a wide area and over a long time. The present invention is a method for forming a thin film containing a metal oxide as the main component on a substrate using a mixed gas stream containing a metal chloride, an oxidizing material, and hydrogen chloride, by a thermal decomposition method at a film deposition rate of 4500 nm/min. or greater, performing at least one selected from: 1) prior to mixing the metal chloride and the oxidizing material in the mixed gas stream, contacting hydrogen chloride with at least one selected from the metal chloride and the oxidizing material, and 2) forming a buffer layer in advance on a surface of the substrate on which the thin film containing a metal oxide as the main component is to be formed.