Sulfonated polypropylene separators impregnated with the polymer hydrogel electrolyte were used in electric double layer capacitors (EDLCs). The electrochemical properties of the EDLC with the polymer hydrogel electrolyte were investigated by cyclic voltammetry and charge–discharge cycle tests and compared with a KOH aqueous electrolyte. Furthermore, effects of KOH concentration and temperature on capacitance of the EDLC were studied. As a result, it was found that the capacitance of the EDLC with the polymer hydrogel electrolyte was higher than that with a KOH aqueous solution in the wide range of KOH concentration and temperature.