The edges of portions of a base member that are joined to stem pins are arranged as bottom surfaces of recesses formed in the stem so that the stem pins are joined to the base member at gradual angles and so that even when a bending force acts on the stem pins, the stem pins will contact the peripheral portions at the open sides of the recesses, thereby preventing further bending of the stem pins and preventing the forming of cracks at both sides of the portions at which the stem pins are joined to the base member. Furthermore, triple junctions, at which the conductive stem pins, the insulating base member to which the stem pins are joined, and vacuum intersect, are positioned inside the recesses and put in concealed-like states.