Provided are a composition for forming film which can form porous film excelling in dielectric constant, adhesiveness, uniformity of the film, mechanical strength and having low hygroscopicity; 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, the composition comprising siloxane polymer and one or more quaternary ammonium salts represented by following formula (1) or (2): [(R.sup.1).sub.4N].sup.+[R.sup.2X].sup.- (1) H.sub.k[(R.sup.1).sub.4N].sub.m.sup.+Y.sup.V- (2) wherein X represents CO.sub.2, OSO.sub.3 or SO.sub.3; Y represents SO.sub.4, SO.sub.3, CO.sub.3, O.sub.2C--CO.sub.2, NO.sub.3 or NO.sub.2; and k is 0 or 1, m is 1 or 2 and n is 1 or 2 in proviso that n=1 requires k=0 and m=1, and n=2 requires k=0 and m=2, or k=1 and m=1.