PURPOSE: To eliminate the sag of a display background and maintain superior whiteness with a large-sized ECD cell as well by providing a porous polyvinyl formal resin plate impregnated with a resin between display electrodes and a counter electrode.

CONSTITUTION: A porous white display backgrund plate 1 made by impregnating a melamine resin or epoxy resin in a porous polyvinyl formal plate (0.17 apparaent specific gravity, about 86% porosities) then heating the same to set is provided between the display electrodes 10 and counter electrode 3 of an electrochromic display (ECD) device. This background plate 1 has open cells, has good solvent resistance, does not contain any inhibitive material to be eluted into electrolyte 4, has moderate hardness and good workability, is hard to break, is inexpensive and will not swell with org. electrolyte or the like. Hence, superior display quality is obtained and the device is suited for mass production.