PURPOSE: To prevent an iron metal layer and a high melting point metal conductor layer from being oxidized by a method wherein a conductive paste layer containing at least one or more of copper, nickel or cobalt as its main component and added a reducing agent of silicon, boron, or a compound consisting of these elements is provided between the iron metal layer and a thick film conductor layer.

CONSTITUTION: A conductive paste layer 16' containing copper, cobalt or nickel as its main component and containing a reducing agent is subjected to screen printing on an iron plated layer and moreover, a thick film conductive paste layer 17' containing a base metal, such as copper, as its main component is formed on both of the layer 16' and an insulating layer 14 by printing into a necessary circuit pattern and thereafter, the layer 17' is fired in an atmosphere containing such inert gas as nitrogen gas to obtain an oxidation-resistant protective layer 16 and a thick film conductor layer 17. In case silicon, for example, is used as the reducing agent, its content is set in 1.5∼3.0wt.% or thereabout and in case manganese boride is used, its content is set in 1.5∼3.0wt.% or thereabout. A simultaneous firing becomes possible by this reducing agent and the iron group metal layer and a high-melting point metal conductor layer 12 are prevented from being oxidized.