To provide a composition for forming an insulation film which is excellent in film properties such as permittivity and Young's modulus, capable of forming a film having a proper uniform thickness and suitable for using as an interlayer insulation film of semiconductor devices and the like.

The composition is formed by dissolving a cage type silsesquioxane compound in an organic solvent at a concentration of 12 mass%, which silsesquioxane compound has two or more unsaturated groups as substituents, and polymerizing the compound by using a polymerization initiator, wherein the composition is characterized by containing a polymer provided by polymerizing 70% of the compound and the polymerization initiator is preferably an azo compound.