A cathode foil for a solid electrolytic capacitor is designed to increase capacitance, reduce ESR and leakage current, enhance heat resistance, and reduce production costs, while enhancing a power density, realizing rapid charging-discharging, and improving a life property, in an electric energy storage element such as a secondary battery, an electric double layer capacitor and a hybrid capacitor. A cathode foil or a current collector may include a metal foil, a metal layer, a mixed layer containing carbon and a substance composing the metal layer in a mixed state, and a carbon layer consisting substantially of carbon, each formed on the metal foil. The mixed layer is configured to have a composition changing from a state containing substantially only the substance composing the metal layer to a state containing substantially only carbon, in a direction from the metal layer to the carbon layer.