An object of the present invention is to provide a composite of silicon oxide nanoparticles and a silsesquioxane polymer, from which a cured film having a low refractive index can be formed inexpensively. Provided are a method of producing a composite of silicon oxide nanoparticles and a silsesquioxane polymer, the method comprising reacting a silsesquioxane polymer having a silanol group at a terminal or a silane monomer with silicon oxide nanoparticles having a hydroxyl group or an alkoxy group on the surface in a mixed solvent of an aqueous solvent and an organic solvent in the presence of a phase transfer catalyst, and a composite produced by the method.