PURPOSE: To obtain a circuit base with low dielectric const. and dielectric loss, no water permeation from the base board surface to the inside and uniform dielectric characteristics by laminating a metal foil or a metal plate on at least one face of a porous insulator layer through a water-impermeable membrane and sealing the end face of the surroundings of the plate with a resin.

CONSTITUTION: Metal foils 3 are adhered on both faces of a porous insulator layer 1 through a water-impermeable membrane 2 and the end face is sealed with a sealing resin 4. The insulator layer 1 is prepd. by adhering or fusing a resin foam or a resin particle and molding simultaneously so as to leave gaps of particle filling. As the metal foil or the metal plate, a foil or a plate made of copper, nickel, bronze, brass, aluminum, nickel, iron, stainless steel, gold, silver, platinum etc., can be used. Before circuit processing of the circuit base thus constituted, the part of the base end not to be used as a circuit is sealed with a curable liq. resin. As the method for sealing, a plurality of circuit base boards with the same shape and size are laminated and the edges thereof is immersed in a resin soln. in a pat-shaped container or is coated with the resin soln. and solidified and cured.