To reduce useless power consumption and to prolong the service life of cells by reducing impacts which degrade the cells.

A lighting ratio which is the ratio of the number of the cells to be lighted to the total number of the cells is detected on the basis of display data determining the contents of addressing, In accordance with the detected lighting ratio, a waveform of a voltage pulse that is applied in a sustaining step for displaying the pertinent display data is changed so that a gradient of voltage change at a leading edge becomes smaller for a large value of the lighting ratio than for a small value of the same.