A solid electrolytic capacitor comprises an electrically conducting polymer composition formed on the surface of an oxide film which is formed on a valve-acting metal, by specifying the viscosity of an oxidizing agent solution and/or a monomer solution, particularly by specifying the viscosity to less than about 100 cp. A solid electrolytic capacitor can comprise an electroconducting polymer composition formed on the surface of an oxide film layer which is formed on a valve-acting metal, wherein a monomer compound or a derivative thereof as a repeating unit is polymerized by setting the humidity in the atmosphere to from about 10% to less than about 60%.