PURPOSE: To improve a multilayer substrate of this design in an electrical property and to decrease restrictions placed on the packaging of components by a method wherein a thermoplastic resin precursor is applied in twice, where the thermoplastic resin precursor is filled into a viahole through a first application and then 7 the precursor possessed of a specified thickness is formed through a second application.

CONSTITUTION: A conductor pattern 32 is formed on an insulating substrate, a polyimide precursor 38 is applied on the insulating substrate on whose surface the conductor pattern 32 has been formed, the polyimide is transferred into glass by heating after a viahole 42 has been provided to a specified position on the precursor 33 through a photolithography, and a conductor pattern is formed thereon. A precursor 33 of low viscosity is applied to be filled into the viahole 42 and a polyimide precursor, whose viscosity is so set as to be adequate for forming a layer with a required thickness, is applied, The formation of the viahole, heating, the formation of the conductor pattern, and the application of the polyimide in twice are repeated two or more times, and a polyimide precursor is applied onto an uppermost layer twice, which is heated. Voids are prevented from being generated inside, the layer is uniform in quality, and an impedance property can be obtained as specified by a design.