Anodized films are formed at both surfaces of an aluminum base and, at the center portion on each side of the aluminum base, a solid electrolyte layer of a conductive polymer, a graphite layer, and a metal layer are stacked in the order, thereby forming a rectangular cathode portion. An insulator layer is formed at the peripheries of four sides of the cathode portion and, further, an anode lead frame is provided at the peripheries of four sides of the upper insulator layer, thereby forming an anode portion. Openings are formed at four corners of the insulator layer or the anode portion, thereby establishing electrical connection between the cathode portions on both sides of the aluminum base. By setting the ratio of a total region, occupied by the openings, of the cathode portion to 25% or less, the ESL of a capacitor can be suppressed low even when the openings are provided.