A solid electrolyte capacitor superior to a manganese dioxide capacitor and characterized by the use of an oxide of Ru, Rh, Re, Os or Ir either in place of or together with manganese dioxide. The newly utilized metal oxide is good at oxidizing ability and low in resistivity. A solid electrolyte layer comprising the newly utilized metal oxide is formed by pyrolysis of an aqueous solution comprising a soluble salt of the metal on an anodized valve metal. The solution may comprise manganese nitrate too.