The present invention relates to a solid electrolytic capacitor having a masking structure in which the insulation between the anode part and the cathode part can be ensured without fail, to its production method, to a method for coating a masking agent on a solid electrolytic capacitor substrate, and to apparatus therefore. According to the present invention, the masking material covers the dielectric film on the metal material having valve action and sufficiently infiltrates into the core metal made of a metal having valve action while the solid electrolyte is masked by the masking material without fail, so that a solid electrolytic capacitor can be produced that has a reduced leakage current and a reduced stress generated at the reflow treatment or the like.