A multilayer wiring substrate includes a substrate main body and a plurality of wiring lines. The substrate main body includes first and second main surfaces. The plurality of wiring lines extend from the first main surface toward the second main surface side in the substrate main body. The substrate main body includes a plurality of insulator layers laminated on each other. The wiring lines each include via conductors separately provided in the plurality of insulator layers. In at least one of the plurality of wiring lines, a diameter of the via conductor provided in a first insulator layer defining the first main surface of the substrate main body is smaller than a diameter of the via conductor provided in at least one of the plurality of insulator layers other than the first insulator layer.