A method for forming conductor layers of substrates for mounting LSIs and the like and a fabrication method of multilayer substrates are disclosed. These methods comprise steps of forming a metal underlayer having a shape similar to that of a conductor pattern on the substrate, forming an insulation layer over portions of the substrate which are not covered by the metal underlayer, and disposing a plating layer on the metal underlayer by carrying out electroless plating while using the insulation layer as the resist and thereby forming conductors. As compared with a conventional conductor layer forming method, the number of fabrication steps is reduced. And the elimination of the surface grinding step facilitates the fabrication.