A method of forming a composite insulating layer in which a thin and uniform adhesive layer can be formed, and an adhesive strength between an adhesive layer and a porous layer is sufficient, a insulating layer obtained by the formation method, and a process of producing a wiring board, wherein an insulating layer is formed by the formation method, are disclosed. The method of forming a composite insulating layer includes a step of forming an adhesive layer 2 containing a partially imidated polyamic acid on a metallic foil 1; a step of applying a solution 3 containing a polyamic acid to the surface of the adhesive layer 2; a step of subjecting the coated solution 3 to phase separation to form a porous layer 4; and a step of subjecting the adhesive layer 2 and the porous layer 4 to imide conversion.