Each of the insulator layers is formed by firing a ceramic green sheet, and may include in its surface a void V formed due to expansion and contraction of ceramic upon firing. The void V is a depression in the thickness direction of the insulator layers, and is exposed on a surface of the each of the insulator layers: especially, on a surface of the ceramic board 2. The void V has a maximum width of approximately 30 μm in planar view.

The ceramic insulator layers may be made of any kind of ceramic such as alumina, low temperature co-fired ceramic (LTCC), and middle temperature co-fired ceramic (MTCC). LTCC is especially preferable.

Employment of LTCC serves to reduce a temperature for firing the ceramic board 2, and allows copper alloy such as Cu—W to be employed for the wiring of the ceramic board 2. This serves to reduce a production cost of the ceramic board 2 and improve the wiring in conductivity. On the other hand, LTCC tends to undergo comparatively large expansion and contraction upon firing, and is likely to include a void. The present disclosure serves to suppress the electrode pad 4 from undergoing the bulge, owing to through holes 4A described below.

<Electrode Pad> The electrode pad 4 is electrically connected to the conductor 3, and is disposed on the surface of the ceramic board 2: in other words, on a surface of the insulator layer 21 being an outmost layer of the ceramic board 2.