In order to ensure high levels of insulation performance, and low cost, and have an excellent external appearance, as a ceramic constituting an insulating coating 6 being a ceramic coating layer, one with alumina as its main component, and in which the content of titanium oxide is limited to 0.01 to 0.2 percent by weight is used. Moreover, a particle size of ceramic forming the coating layer is 10 to 50 μm, and average particle size is 15 to 25 μm. By limiting the content of titanium oxide, the insulating coating 6 can be made thin, and by limiting the particle size the film thickness accuracy can be improved, thus simplifying finishing. Moreover, by mixing of titanium oxide, deterioration of the external appearance after sealing can be prevented. As a result, the above problem is solved.