A method for forming a porous silica film having mechanical strength utilizeses a surfactant, one or more kinds of nonionic surfactant(s) having a 0.1 weight % concentration according to the Du Nouy method expression and a surface tension of 45 mN/m or larger at 25 C. is (are) used as a surfactant, a mixed solution obtained by mixing this nonionic surfactant, a hydrolyzable alkoxysilane compound, water and an alcohol is coated on the substrate, and the surfactant in this mixed solution is decomposed or burned out to form a porous silica film. The surfactant is suitably represented by formula OH(CH₂CH₂O)x(CH(CH₃)CH₂O)y(CH₂CH₂O)xH where x and y denote an integer satisfying 1x185 and 5y70, respectively. Alternatively, a mixed solution in which a dimethyldialkoxysilane is added may be used.