A positive side terminal electrode and a negative side terminal electrode which are exposed to the exterior of a resin enclosure pass through a positive side opening and a negative side opening formed on the enclosure, respectively, and are connected to a positive side electrode and a negative side electrode, respectively, of an electric double-layer capacitor put away in the interior of the enclosure. The positive side terminal electrode is thermally welded to inner surfaces of the positive side opening over the entire periphery thereof, and the negative side terminal electrode is thermally welded to inner surfaces of the negative side opening over the entire periphery thereof.