A panel capacitor is formed by a scan electrode and a sustain electrode. The voltage at the panel capacitor is reduced by turning on a transistor coupled between the scan electrode and the capacitor. As a result, the voltage within the panel capacitor exceeds the discharge firing voltage to discharge the panel capacitor. When the gate voltage of the transistor is reduced by an RC circuit, the transistor is turned off, and the scan electrode is floated. A discharge is then steeply quenched, and wall charges are finely controlled. Next, the above-noted operation is repeated by turning on the transistor.