A method for forming a thin film, which comprises the steps of forming a cluster which is a lumpy group of atoms or molecules of a reactive substance gaseous at the room temperature, irradiating cluster ions ionized therefrom onto a substrate surface, and at the same time or alternately, irradiating a single, or a plurality of, component gas of a deposit film onto the substrate surface to cause reaction of the both, thereby depositing a thin film on the substrate surface;
- wherein two or more gases to be irradiated simultaneously are fed after converting same into clusters.