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PROBLEM TO BE SOLVED : To obtain a double-layer capacitor having a large capacitance and moreover superior durability by a method, wherein the solvent of an electrolyte is mainly composed of the mixture of propylene carbonate and ethylene carbonate, and the natural potential of a polarized electrode body is set at specific voltage or lower. SOLUTION : A mixture of propylene carbonate and ethylene carbonate is used mainly as the solvent of an electrolyte, and the spontaneous potential in the electrolyte of a polarized electrode is set between 1.7 and 2.7 V (Li/Li ). At this point, the polarized electrode is composed of a carbonaceous electrode and at least one or more substances selected from inorganic substances. Also, the substance, containing an alkaline metal element, an alkaline earth metal and a rare-earth metal, especially the substance selected at least from one or more substances containing lithium element, is used.