The invention provides a coating solution for forming a porous film having excellent mechanical strength and dielectric properties and for easily forming a film with a freely controlled film thickness in an ordinarily employed method in semiconductor process. More specifically, provided are a method for preparing a porous-film-forming composition comprising steps of preparing polysiloxane, silica or zeolite particles (Component A), imparting crosslinkability to Component A, and temporarily terminating the crosslinkability; and a porous-film-forming composition obtainable in this method. In addition, provided is a method of forming a porous film comprising steps of preparing a porous-film-forming composition by preparing Component A, imparting crosslinkability to Component A and adding a crosslinkability inhibitor to temporarily terminate the crosslinkability; applying the porous-film-forming composition onto a substrate to form a film, drying the film, crosslinking the particles along with removing the crosslinkability inhibitor by heating the dried film.