There are provided a silane polymer having a higher molecular weight from the viewpoints of wettability when applied to a substrate, a boiling point and safety, a composition which can form a high-quality silicon film easily, a silicon film forming composition which comprises a silane polymer obtained by irradiating a photopolymerizable silane compound with light of specific wavelength range to photopolymerize it, and a method for forming a silicon film which comprises applying the composition to a substrate and subjecting the coating film to a heat treatment and/or a light treatment.