Abstract: A flat matrix cathode-ray tube (CRT) in which the brightness of a display picture can be readily adjusted according to environmental brightness. In the flat matrix CRT, the grid electrodes are made up of the scanning electrodes and the data electrodes which form the matrix structure with the scanning electrodes, and the pulse voltage for gradations required are applied selectively to the scanning electrodes and the data electrodes so that the picture elements determined by the scanning electrodes and the data electrodes to which the pulse voltage are selectively applied are caused to emit light. Slots the number of which is determined in correspondence to the number of gradations selected, are provided either in the waveforms of the voltages applied to the scanning electrodes or in the waveforms of the voltages applied to the data electrodes in such a manner that the slots occur at regular intervals.