There is disclosed a thermosetting composition for forming a silicon-containing film to form a silicon-containing film formed in a multilayer resist process used in lithography, including at least (A) a silicon-containing compound obtained by hydrolyzing and condensing a hydrolyzable silicon compound using an acid as a catalyst, (B) a thermal crosslinking accelerator (C) a monovalent or bivalent or more organic acid having 1 to 30 carbon atoms, (D) trivalent or more alcohol and (E) an organic solvent. There can be provided a composition for a silicon-containing film which can form a good pattern in a photoresist film, can form a silicon-containing film for an etching mask having a good dry etching resistance, can give a good storage stability and can be delaminated with a solution used in a delamination process in a multilayer resist process used for lithography, a substrate on which the silicon-containing film is formed, and further a method for forming a pattern.