According to the present invention, a method for depositing an ultra-fine crystal particle polysilicon thin film supplies a source gas in a chamber loaded with a substrate to deposit a polysilicon thin film on the substrate, wherein the source gas contains a silicon-based gas, an oxygen-based gas and a phosphorous-based gas. The mixture ratio of the oxygen-based gas to the silicon-based gas may be 0.15 or lower (but, excluding zero). Oxygen in the thin film may be 0.8 atomic percent or lower (but, excluding zero).