Driving electrolytes for electrolytic capacitors of the present invention contain boric acid esters and/or their salts of the structural formulae: Rt—[(R1)1—O—B—(OH)—O]k—H;  (1) Rt—[(R1)1—O—B—(OH)—O—(R2)n]k—Rt; and  (2) In the foregoing formulae, Rt represents CH3 or H ; R1, R2 and R3 represent —CH2O, —C2H4O, —C3H6O, or —C4H8O; and K, l, and m indicate arbitrary natural numbers. Other electrolytes of the present invention further include inorganic acids and/or organic acids and have boric acid esters and/or their salts of the foregoing structural formulae dissolved in the solvent thereof. Using the above-mentioned electrolyte, an electrolytic capacitor which has high withstand voltage of not less than 500V even at the temperatures of 125° C. can be obtained.