A circuit substrate is provided with a base formed of ceramics. It includes a first face a second face; and a through hole penetrating from the first face to the second face; a through conductor: containing silver and copper as main components; disposed inside the through hole; and including a plurality of surfaces; and a metal layer in contact with at least one of the plurality of surfaces. The through conductor includes a eutectic region of silver and copper, disposed in a metal layer side of a diametrically center region of the through conductor; and a non-eutectic region of silver and copper, disposed in a central region of the diametrically center region of the through conductor.