NOVELTY - A nonaqueous electrolyte solution comprises an electrolyte and a nonaqueous solvent. The nonaqueous electrolyte solution comprises a compound having two or more isocyanate groups in a molecule, and a compound having two or more cyano groups in a molecule. The mass ratio of compound having isocyanate groups and compound having cyano groups is 50:50-1:99. The nonaqueous electrolyte solution is used in a nonaqueous electrolyte battery provided with a positive electrode and a negative electrode which occludes/releases a metal ion.    USE - Nonaqueous electrolyte solution is used for nonaqueous electrolyte batteries (claimed). Uses include but are not limited to notebook personal computers, pen input personal computers, electronic book players, mobile telephones, portable FAX, portable copiers, portable printer, video movies, liquid-crystal televisions, minidiscs, transceivers, electronic organizers, calculators, memory cards, portable tape recorders, backup power supplies, motors, motor vehicles, motorbikes, lighting fixtures, toys, game machine devices, timepieces, electric tools, electronic flashes, cameras, power supplies used for load leveling, and natural energy storage power supplies.    ADVANTAGE - The nonaqueous electrolyte solution prevents deterioration of battery capacity and gassing even when stored at high temperature. The electrolyte solution provides batteries having excellent cycle characteristics and charging/discharging characteristics under high current density.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for nonaqueous electrolyte solution secondary battery provided with the nonaqueous electrolyte solution, a positive electrode which discharges/occludes lithium ion, and a negative electrode which discharges/occludes lithium ion. The electrolyte is dissolved in nonaqueous solvent. The negative electrode contains a carbonaceous material. The nonaqueous electrolyte solution contains 0.001-5 %mass monofluorophosphate or difluorophosphate.