PURPOSE: To enhance the thermal resistance and humidity resistance of an insulating layer by a method wherein the surface of the insulating layer is smoothed out using a glaze further leaving the least fine voids in the insulating layer.

CONSTITUTION: The granulated particle containing zinc oxide is pressure-molded and temporary-baked to produce a calcined body 1. Next, this calcined body 1 with at least a part thereof coated with an insulating material 2 is regular- baked. At this time, this insulating material 2 contains at least one element out of zinc oxide, silicon oxide, antimony oxide and bismuth oxide as well as a glaze component 3. Furthermore, after coating the surface of the calcined body 1 with the insulating material 2 and then the glaze component 3, the regular baking step may be performed.