A capacitor that includes a conductive porous base material; a dielectric layer; and an electrode. The conductive porous base material, the dielectric layer, and the upper electrode are laminated together to constitute an effective part that accumulates charges in the dielectric layer when a voltage is applied between the conductive porous base material and the electrode. The conductive porous base material includes at least one groove having a width of 10 μm or more at ½ of a depth of the at least one groove.