As shown in FIG. 1, the electrode pad 4 further includes a central part 41 in which the through holes 4A are formed, and a peripheral part 42 surrounding the central part 41 without involving the through holes 4A.

The central part 41 involves a center such as a geometrical center of the electrode pad 4 in planar view (specifically, view in a thickness direction of the electrode pad 4), and spreads to overlap with the conductor 3. According to the present embodiment, the central part 41 is exemplified to be a circular part centered at the center of the electrode pad 4.

The central part 41 has a width that is defined as a diameter of a minimum circle enclosing the central part 41 and is suitably set depending on a diameter of the conductor 3 and on a size and an expansion/contraction rate of the ceramic board 2. The peripheral part 42 is a part out of the central part 41 in planar view of the electrode pad 4, and includes an outer edge of the electrode pad 4.

According to the present embodiment, the through holes 4A have a same shape, and are arranged in grid at equal intervals in the central part 41: in other words, arranged to be aligned lengthwise and breadthwise at equal intervals. As shown in FIG. 1, the central part 41 includes at its center a portion in which none of the through holes 4A is formed, for increasing the central part 41 in area in connection with the conductor 3. Incidentally, the through holes 4A may be arranged in zigzag, for example.