There is disclosed a thermosetting silicon-containing antireflection film-forming composition, to form a silicon-containing antireflection film in a multilayer resist process used in a lithography, wherein the composition is at least capable of forming—on an organic film that is an underlayer film having a naphthalene skeleton—a silicon-containing antireflection film whose refractive index “n” and extinction coefficient “k” at 193 nm satisfy the following relationship: 2n−3.08≦k≦20n−29.4 and 0.01≦k≦0.5. There can be provided, in a multilayer resist process used in a lithography, a thermosetting silicon-containing antireflection film-forming composition to form a silicon-containing antireflection film which can form an excellent pattern having depressed reflection of an exposing light at the time when a photoresist film is formed on the silicon-containing antireflection film formed on an organic film having a naphthalene skeleton as a resist underlayer film and subsequently a resist pattern is formed; has excellent dry etching properties between the photoresist film—which is the upperlayer of the silicon-containing antireflection film—and the organic film—which is the underlayer—; and has an excellent storage stability, and a substrate having the silicon-containing antireflection film from the composition for forming the silicon-containing antireflection film, and a patterning process using the same.