The electroconductive portion has a structure including: an electroconductive phase, which is made of a metal component containing at least one of W or Mo and has a plurality of air gaps dispersed so as to be separated from each other; and glass phases, which are contained in at least some of the plurality of air gaps and have an area ratio of 3% or more and 20% or less on a cross section of the plate-shaped portion taken in a thickness direction. A proportion of the number of the glass phases, each having an aspect ratio of 1.5 or less, to a total number of the glass phases on the cross section of the plate-shaped portion taken in the thickness direction is 40% or more.