Disclosed is a method for producing a porous silica or porous silica film which has low relative dielectric constant and high mechanical strength and is suitably used for optical functional materials, electronic functional materials and the like. Also disclosed are methods for producing an interlayer insulating film, material for semiconductors and semiconductor device respectively using such a porous silica film, and apparatuses used for producing those. A composite is obtained by drying a solution containing a hydrolysis-condensation product of an alkoxysilane and a surface active agent, and the composite is sequentially subjected to an ultraviolet irradiation treatment and a hydrophobilizing treatment using an organosilicon compound having an alkyl group in this order. A porous silica film can be obtained by drying the solution on a substrate for forming the composite.