In an aging process in which a voltage having an alternate voltage component is applied to at least between a scan electrode and a sustain electrode so as to form a discharge dent (sputter dent) on a protecting layer, the aging discharge dent is formed so as to satisfy any one of the following. First, the discharge dent on the scan electrode-side has a width which is narrower than the discharge dent on the side of sustain electrode. Second, the discharge dent on the side of sustain electrode is formed so that the depth of the discharge dent in the area away from a scan electrode paired with a sustain electrode as a display electrode is shallower than the depth of the discharge dent in the area close to counterpart scan electrode.