An electric double-layer capacitor has at least one or two elements. Each element includes a charge storage part, a pair of collectors arranged to hold the charge storage part, and a gasket enclosing the charge storage part and being arranged between the collectors to be thermally adhered to the collectors. Each collector is formed from a conductive polyethylene sheet which is provided with conductivity by a carbon material and has a multi-layer structure including polyethylene materials of different density levels.