An object of the present invention is to provide a porous material (resin composition) having high heat insulation properties, mechanical properties, and electrical properties by controlling function of a porous film by setting a porosity size, distribution of the porosity size, and a porosity ratio of the porous film in predetermined ranges. The resin composition according to the present invention is comprised of an engineering plastic having porous structure in which not less than 80% of a total porosity is comprised of independent porosities, a mean porosity size is not less than 0.01 μm and not more than 0.9 μm, and not less than 80% of the total porosity has a porosity size within ±30% of the mean porosity size.