Provided are a composition for forming film which can form a porous film excelling in dielectric constant, adhesiveness, uniformity of the film, mechanical strength and having low hygroscopicity; a porous film and a method for forming the film; and a high-performing and highly reliable semiconductor device comprising the porous film inside. More specifically, provided is a composition for forming porous film, comprising a surfactant and a solution comprising polymer obtainable by hydrolyzing and condensing, in the presence of the surfactant, one or more of alkoxysilane represented by Formula (1) and one or more of alkoxysilane represented by Formula (2): (R.sup.1).sub.mSi(OR.sup.2).sub.4-m (1) R.sup.3Si(R.sup.4).sub.n(OR.sup.5).sub.3-n (2) Also provided is a method for forming porous film comprising a step of applying said composition on a substrate to form film and a step of transforming the film into porous film.