Provided are a composition for forming porous film which can form a porous film having practical mechanical strength in a simple and low cost process; a porous film and a method for forming the film; and an inexpensive, high-performing and highly reliable semiconductor device comprising the porous film inside. More specifically, provided is a composition for forming porous film, comprising a polymer which is obtainable by hydrolyzing and condensing one or more silane compounds represented by Formula (1), or preferably by hydrolyzing and co-condensing one or more silane compounds represented by Formula (1) and one more silane compounds represented by Formula (2), Formulas (1) and (2) being: (R.sup.1).sub.aSi(R.sup.2).sub.4-a (1) (R.sup.3).sub.bSi(R.sup.4).sub.4-b (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.