A silicon-containing polymer which results in a composition that forms a coated film having good adhesive properties to a substrate and does not easily peel off the substrate, a film-forming composition including the polymer, a method for forming a silicon-containing polymer coating using the composition, a method for forming a silica-based coating using the composition, and a production method for the polymer. In a silicon-containing polymer including at least one of poly- or oligo-siloxane chain and poly- or oligo-silane chain in a molecular chain, a group having a silyl group and a sulfide group, which can be subjected to a condensation reaction, is introduced into the molecular chain, for example, by an ene-thiol reaction.