PURPOSE: To reduce the unevenness of coloring density generated between respective segments by adding an AC voltage source for adjusting coloring density.

CONSTITUTION: An AC voltage is impressed to adjust the density to a set value during a time T2. The impression of the AC voltage makes it possible to reduce the sensity of a segment thicker than the set value and increase that thinner than the set value, so that dispersion of density between segments and between elements is reduced. In addition, almost constant coloring density can be obtained even at a high or low temperature. The coloring voltage Vc is varied in accordance with an electrochromic (EC) substance of a display pole and a substance of an opposite pole. In case WO3, the coloring voltage Vc may be usually about 1.5V and the coloring voltage application time T1 is about 0.1W1min in accordance with a dispaly segment area, coloring density, lead resistance, etc.