PURPOSE: A method for forming a composite thin film is provided to easily form a composite thin film with a wanted composition ratio by efficiently controlling a generation ratio of an absorption site on an object. CONSTITUTION: A first precursor including a first central element and a ligand combined with the first central element is absorbed on a first absorption site of an object(S10). The ligand or first central element of the absorbed first precursor is formed by removing the ligand or first central element of the absorbed first precursor(S20). A composite thin film is formed on the object by absorbing a second precursor on a second absorption site(S30). The composite thin film is formed on the substrate by introducing a reactive material to react with the first precursor and the second precursor(S40).
