An interconnection substrate includes: a substrate having a first surface and a second surface opposite the first surface; and a transmission line including two parallel through-hole interconnections that are exposed to the first and second surfaces and are formed inside the substrate. Also, at least one of the two through-hole interconnections includes a narrow portion having a smaller diameter than a diameter of the through-hole interconnection in the first surface and a diameter of the through-hole interconnection in the second surface.