The object of the invention is to provide a porous film having the dielectric constant of 2.2 or less and having practicable mechanical strength. This invention provides a porous film-forming composition comprising (A) and (B):

• (A) 100 parts by weight of a hydrolyzable silicon compound and/or a product resulting from hydrolysis condensation of silicon compound expressed by following formula (1):

•  wherein Z¹ denotes a hydrolyzable group; R¹ denotes a substituted or non-substituted monovalent hydrocarbon group; and a denotes an integer of 0 to 3; and
• (B) 0.1 to 20 parts by weight of a cross-linking agent comprising at least one cyclic oligomer which can generate silanol group(s) by heating and which is expressed by following formula (3):

•  wherein R³¹ and R³² each denotes a substituted or non-substituted monovalent hydrocarbon group; Z³ denotes a group which can generate silanol by heating; and each d and e denotes an integer of 0 to 10, and a sum of d and e is greater than or equal to three.