To detect a dark current when an abnormal discharge occurs between an anode and respective electrodes, on an inner surface of a front substrate, a dark current detection electrode is formed in a state that the dark current detection electrode is positioned adjacent to the outside of a screen display region on which an anode is formed and on a plane substantially equal to a plane on which the anode is formed. Then, between an electrode terminal of the dark current detection electrode and a ground, an ammeter which detects the flow of a dark current and a DC power source having a preset voltage value which is more or less lower than a high voltage supplied to the anode are connected in series.