Abstract: An aluminum foil type solid electrolytic capacitor wherein the distance between the two foils in the capacitor element to be determined by the thickness of separator being kept at a value between ten to sixty micrometers, solid electrolyte being formed between the two foils by the thermal decomposition of electrolytic solution impregnated in the capacitor element.Manufacturing methods of making an aluminum foil type solid electrolytic capacitor with a manganese dioxide electrolytic layer between the electrode foils, wherein the layer is formed by decomposing the electrolytic solution thermally under conditions of a temperature between 200 and 260.degree. C. and of a time interval between 20 and 40 minutes, wherein the layer is formed by decomposing electrolytic solution with fine powders of manganese dioxide, wherein lithium is doped in the layer or wherein electrochemical conversion processes are used for restoring an aluminum oxide film.