The present invention provides a zeolite sol which can be formed into a porous film that can be thinned to an intended thickness by a method used in the ordinary semiconductor process, that excels in dielectric properties, adhesion, film consistency and mechanical strength, and that can be easily thinned; a composition for film formation; a porous film and a method for forming the same; and a high-performing and highly reliable semiconductor device which contains this porous film inside. More specifically, the zeolite sol is prepared by hydrolyzing and decomposing a silane compound expressed by a general formula: Si(OR.sup.1).sub.4 (wherein R.sup.1 represents a straight-chain or branched alkyl group having 1 to 4 carbons, and when there is more than one R.sup.1, the R.sup.1s can be independent and the same as or different from each other) in a conventional coating solution for forming a porous film in the presence of a structure-directing agent and a basic catalyst; and then by heating the silane compound at a temperature of 75.degree. C. or lower. A composition for forming a porous film which contains this zeolite sol is used.