NOVELTY - An electrolyte solution contains an electrolyte (B) containing a quaternary ammonium salt (A) (I), a nonaqueous mixed solvent (H) and a nonaqueous solvent (G). The non-aqueous mixed solvent (H) contains sulfolane derivative (S) chosen from sulfolane, 3-methylsulfolane and 2,4-dimethylsulfolane. The nonaqueous solvent (G) is chosen from ester (G1) (II), benzene derivative (G2) (III) having molecular weight of 400 or less and nitrile derivative (G3) (IV).    USE - Electrolyte solution is used for electrochemical element, electrochemical capacitor and electrical double layer capacitor (all claimed). Can also be used in secondary battery and dye-sensitized solar cell.    ADVANTAGE - The electrolyte solution provides electrochemical element having excellent stability, high withstand voltage property and durability.    DETAILED DESCRIPTION - An electrolyte solution contains an electrolyte (B) containing a quaternary ammonium salt (A) of formula (I), a nonaqueous mixed solvent (H) and a nonaqueous solvent (G). The non-aqueous mixed solvent (H) contains sulfolane derivative (S) chosen from sulfolane, 3-methylsulfolane and 2,4-dimethylsulfolane. The nonaqueous solvent (G) is chosen from ester (G1) of formula: R15C(=O)OR16 (II), benzene derivative (G2) of formula (III) having molecular weight of 400 or less and nitrile derivative (G3) of formula: R20-CN (IV).    R1=1-10C monovalent hydrocarbyl optionally having group chosen from halo, hydroxy, nitro, cyano and ether linkage;    R2=H, halo or 1-10C monovalent hydrocarbyl optionally having group chosen from halo, nitro, cyano and ether linkage;    R3-R14=1-5C alkyl, 1-5C fluoroalkyl, H or halo;    h-j,x-z=0-6;    h+x=0-6;    i+y,j+z=1-6;    X-=counter anion having highest occupied molecular orbital energy of -0.60 a.u to -0.20 a.u by first principle molecular orbital calculation;    R15,R16=1-10C aliphatic hydrocarbyl;    or R15+R16=ring;    R17=halo or 1-7C organic group;    R18,R19=H, halo or 1-7C organic group;and    R20=1-10C aliphatic hydrocarbyl optionally having 1-2 cyano and ether linkage.