There is disclosed an antireflection film composition for forming an intermediate resist film of a multilayer resist film used in lithography comprising: at least a polymer obtained by reacting a chelating agent with a polymer having a repeating unit represented by the following general formula (1); an organic solvent; and an acid generator. There can be provided a novel antireflection film composition that exhibits high etch selection ratio to a photoresist film, that forms a dense inorganic film, whereby an excellent pattern can be formed on the overlying photoresist film, that can be removed with wet stripping, that exhibits high preservation stability and high dry etching resistance when an underlying layer is etched, and that is suitable for forming an intermediate resist film of a multilayer resist film; a patterning process in which an antireflection film is formed over a substrate by using the antireflection film composition; and a substrate having the antireflection film as an intermediate resist film.