NOVELTY - A nonaqueous electrolyte solution is obtained using electrolyte and nonaqueous solvent which dissolves the electrolyte. The nonaqueous electrolyte solution comprises methyl(oxo)phosphine compound (I), cyclic carbonate having carbon-carbon unsaturated bond, cyclic carbonate having fluorine atom, aromatic compound (II), and compound having two or more isocyanato groups. The content of cyclic carbonate having carbon-carbon unsaturated bond is 0.5 %mass with respect to 100 %mass of nonaqueous electrolyte solution.    USE - Nonaqueous electrolyte solution is used for forming battery (claimed) e.g. lithium-ion secondary battery. Uses include but are not limited to notebook personal computer, electronic book player, mobile telephone, portable printer, headphones stereo, liquid-crystal television, electronic organizer, calculator, memory card, motor vehicle, etc.    ADVANTAGE - The nonaqueous electrolyte solution has excellent stability, and provides battery with excellent cycle characteristics and storing characteristics, high voltage and capacity, and improves initial stage swelling characteristics.    DETAILED DESCRIPTION - A nonaqueous electrolyte solution is obtained using electrolyte and nonaqueous solvent which dissolves the electrolyte. The nonaqueous electrolyte solution comprises methyl(oxo)phosphine compound of formula (I), cyclic carbonate having carbon-carbon unsaturated bond, cyclic carbonate having fluorine atom, aromatic compound of formula (II), and compound having two or more isocyanato groups. The content of cyclic carbonate having carbon-carbon unsaturated bond is 0.5 %mass with respect to 100 %mass of nonaqueous electrolyte solution.    X=organic group containing hetero element;    Y'=alkylenyl, alkenylenyl, alkynylenyl or phenylene (all optionally substituted);    n=1 or 2;    m=0 or 1;    Ra=alkyl, alkenyl, alkynyl, aryl, alkoxy, alkenyloxy, alkynyloxy or aryloxy (all optionally substituted);    M=1-6C hydrocarbyl optionally containing hetero atom, or transition metal of group-13, group-14 or group-15;    Za+=metal ion, proton or onium ion;    a,b=1-3;    l=b/a;    m'=1-4;    n'=1-8;    t=0-1;    p=0-3;    q,r=0-2;    R1=1-20C hydrocarbyl optionally containing hetero atom, halo or X3R4;    R2=1-6C hydrocarbyl optionally containing hetero atom or direct bond;    X1-X3=O, S, or NR5;    R3-R5=H or 1-10C hydrocarbyl optionally containing hetero atom;and    Y'1,Y'2=C, S or Si.    (i) When M is transition metal of group-13, group-14 or group-15, Za+ is metal ion, proton or onium ion, a is 1-3, b is 1-3, l is b/a, m' is 1-4, n' is 1-8, t is 0-1, p is 0-3, q is 0-2 and r is 0-2; (ii) When M is 1-6C hydrocarbyl optionally containing hetero atom, Za+ does not exist, a, b, l and n' are 0, m' is 1, t is 0-1, p is 0-3, q is 0-2, and r is 0-2; (iii) When Y'1 or Y'2 is C or Si, q or r is 0 or 1; and (iv) When Y'1 or Y'2 is S, q and r are 2. An INDEPENDENT CLAIM is included for battery, which comprises negative electrode, positive electrode, and non-aqueous electrolyte solution.