Abstract: An object of the present invention is to provide a capacitor and a capacitor module having a long life and capable of a stable action. Therefore, an electrolytic solution L obtained by dissolving an electrolyte salt having a lower hydrolyzability and a higher reaction potential in an electrode than an amidine salt containing a cation which is an imidazolium in a solvent and a sub solvent that reduces resistance of the electrolytic solution is packed in a cell. The electrolyte salt is a quaternary ammonium salt, the solvent is propylene carbonate, and the sub solvent is dimethyl carbonate. The quaternary ammonium salt is triethylmethylammonium tetrafluoroborate or azacyclobutane-1-spiro-1?-azacyclobutyl tetrafluoroborate. A pressure regulating valve 6 for regulating an inner pressure in the cell is disposed. A portion of the electrolytic solution L to be vaporized during use is packed in the cell as an excessive electrolytic solution in advance.