A method of forming an insulating film includes: forming a polysiloxane insulating film on a substrate; forming a polycarbosilane insulating film on the polysiloxane insulating film; and forming an organic insulating film on the polycarbosilane insulating film. The polysiloxane insulating film is formed by hydrolysis and condensation of a silane compound, and the polycarbosilane insulating film is formed by applying a solution, obtained by dissolving a polycarbosilane compound in a solvent, to the polysiloxane insulating film, and heating the resulting coating,