An insulating film forming composition having good mechanical and insulating properties comprises a silica sol having a good dispersion stability, an organic siloxane polymer and a hydrophobic organic solvent, wherein the silica sol has primary-particles having an average particle size of 5 to 15 nm and secondary-particles having an average particle size of 70 to 100 nm, and the combined amount of said primary and secondary silica particles is in the range of 2 to 50% by weight based on the sum of the silica particles and organic siloxane polymer.