Discharge tubes, each of which includes a glass tube and a ferrule that has a substantially cylindrical shape and is fitted to each end portion of the glass tube, are supported by a plurality of pairs of relay connectors (or supporting members) provided on the front side of a chassis having substantially a plate-shaped configuration. When a discharge tube is supported by relay connectors, stoppers provided on the relay connectors lock the ferrules so that axial movement of the discharge tube relative to the relay connectors is restricted.