The present invention relates to a capacitor having high capacitance, low ESR (equivalent series resistance) in a high-frequency region and low leakage current, including a composite oxide film obtained by reaction of an oxide film obtained by subjecting the surface of the substrate comprising valve-acting metal element to electrolytic oxidation with a solution in which metal ion and an organic base are dissolved and by subsequently sintering the reactant, a solid electrolyte formed on the composite oxide film and a conductor layer formed thereon; a method for producing the same and electronic devices using the same.