PURPOSE: To prevent increase of a leakage current when soldering by reflow is effected by employing a specified organic silicone compound for a separator paper of an organic semiconductor solid state electrolytic capacitor.

CONSTITUTION: A separator paper 3 is wound between both electrode foils, anode foil 1 and a cathode foil 2 which are formed by forming an oxide coat on an aluminum foil and a capacitor element 6 is thus formed. Further, the capacitor element 6 is subjected to heat treatment and the ceparator paper 3 is carbonized, so that the density is decreased. After that, the capacitor element 6 is dipped in a solution in which an organic silicone compound expressed by formula I or II is dissolved after which the solvent is volatilized to introduce an organic silicone compound to the ceparator paper 3. Then, the capacitor element 6 is impregnated with fused TCNQ salt and is rapidly cooled. The open part is sealed by an epoxy resin and electric potential treatment is effected. As a result, an organic semiconductor solid state electrolytic capacitor in which a leakage current is low after soldering by reflow can be obtained.