An SiO2-based film-forming composition giving a protective film which, after impurity diffusion, can be easily stripped off and which has a higher protective effect. The film-forming composition is one for forming a protective film which in the diffusion of an impurity into a silicon wafer, serves to partly prevent the impurity diffusion. This film-forming composition comprises a high-molecular silicon compound and a compound having a protective element which undergoes covalent bonding to the element to be diffused in the impurity diffusion and thereby comes to have eight valence electrons. The protective element is preferably gallium or aluminum when phosphorus is used as the element to be diffused, and is preferably tantalum, niobium, arsenic, or antimony when boron is used as the diffusion element.