Abstract: The solid electrolytic capacitor comprises a sealed capacitor element having, as a solid electrolyte layer, a heat resistant conductive layer formed on a dielectric oxide film. The heat resistant conducting polymer layer contains water, and the capacitor element inside is substantially oxygen-free. A cathode conductive layer is provided on the capacitor element and then both an external cathode lead and an external anode lead are mounted on the capacitor element. Water is supplied to the conducting polymer layer of the capacitor element such that the conducting polymer layer inside contains water. The capacitor element is then sealed such that water is held within the heat resistant conducting polymer layer and that the capacitor element inside is substantially oxygen-free. The step of supplying water is carried out by dipping the heat resistant conducting polymer layer in water or heavy water or exposed to water-containing inert gas.