PURPOSE: To enhance durability against temp. and humidity, by sealing a whole circumference contg. the junctions of cells consisting of plastic bases with a flexible plastic sheet.

CONSTITUTION: Embodiments of plastics usable as a plastic base are polystyrene, polycarbonate, polyester, acrylic resin, epoxy resin, polysulfone, polyethersulfone, etc., and those of a flexible plastic sheet for covering and sealing the whole circumference contg. the junctions of cells are polymethyl methacrylate, polycarbonate, polyethylene terephthalate, etc., preferably having 50W500μm thickness. The sheet is covered by lamination, or dipping the cells in a soln. of this plastics and drying it, or the like method, thus permitting the cells using plastic bases intended for a thin type to be brought into practical use.