NOVELTY - An electrochemical capacitor (200) is provided with an electrolyte, capacitor element (1), and casing. The electrolyte includes cation, anion, a solvent, and a lactone. The capacitor element is provided with a negative electrode (3), positive electrode (2), and separator (4). The negative electrode is provided with an electrode layer that occludes cation. The positive electrode is provided with a polarising electrode layer (2b) and faces the negative electrode. The separator is provided between the positive electrode and the negative electrode.    USE - Electrochemical capacitor is used for hybrid vehicle and fuel-cell vehicle.    ADVANTAGE - The electrochemical capacitor has excellent energy density. The fuel-cell vehicle using the electrochemical capacitor has discharging/charging characteristics.    DETAILED DESCRIPTION - An electrochemical capacitor is provided with an electrolyte, capacitor element, and casing. The electrolyte includes cation, anion, a solvent not comprising a lactone, and a lactone. The capacitor element is provided with a negative electrode, positive electrode, and separator. The negative electrode is provided with an electrode layer that occludes cation. The positive electrode is provided with a polarising electrode layer and faces the negative electrode. The separator is provided between the positive electrode and the negative electrode. The negative electrode, positive electrode, and separator are stacked or coiled. The capacitor element is impregnated with the electrolyte. The casing accommodates the electrolyte and capacitor element. The electrolyte includes lactone (0.001-5 weight%) with respect to the solvent.    DESCRIPTION OF DRAWING(S) - The drawing shows the perspective view of electrochemical capacitor.    Capacitor element (1)    Positive electrode (2)    Collector (2a,3a)    Polarizing electrode layer (2b)    Negative electrode (3)    Negative electrode layer (3b)    Separator (4)    Lead wire (5a,5b)    Electrochemical capacitor (200)