A high voltage proof solid electrolytic capacitor that can prevent deterioration of voltage proof property due to lead-free reflow etc. and a manufacturing method thereof are provided. A capacitor element having an anode and cathode electrode foils wound via a separator is impregnated with a dispersion comprising conductive polymer particles or powder and a solvent to form a solid electrolyte layer consisting of a conductive polymer, and the voids inside the capacitor element having the solid electrolyte layer formed are filled with an ion-conducting substance comprising a mixed solvent comprising ethylene glycol and 3-butyrolactone together with a solute selected from at least one type of an ammonium salt, a quaternary ammonium salt, a quaternized amidinium salt, and an amine salt of an organic acid, an inorganic acid, and a composite compound between organic acid and inorganic acids to obtain a solid electrolytic capacitor.