A method of forming an organic silica-based film, including: applying a composition for forming an insulating film for a semiconductor device, which is cured by using heat and ultraviolet radiation, to a substrate to form a coating; heating the coating; and applying heat and ultraviolet radiation to the coating to effect a curing treatment, wherein the composition includes organic silica sol having a carbon content of 11.8 to 16.7 mol %, and an organic solvent, the organic silica sol being a hydrolysis-condensation product produced by hydrolysis and condensation of a silane compound selected from compounds shown by the general formulae (1): R.sup.1Si(OR.sup.2).sub.3, (2): Si(OR.sup.3).sub.4, (3): (R.sup.4).sub.2Si(OR.sup.5).sub.2, and (4): R.sup.6.sub.b(R.sup.7O).sub.3-bSi--(R.sup.10).sub.d--Si(OR.sup.8).sub.3-c- R.sup.9.sub.c.