Hydrophobicity of a low dielectric constant film comprising a porous silica film is improved by applying a raw material for forming a porous silica film onto a substrate, and performing vapor-phase transport treatment to expose the substrate to an atmosphere of organic amine vapor to which no water is added. Simultaneously, reduction in a dielectric constant, reduction in leakage current, and improvement in mechanical strength are attained by controlling a pore diameter in a predetermined range.