PURPOSE: To achieve good cycle life even under high temperature atmosphere for a nonaqueous electrolyte secondary battery, in which a complex metal oxide including Li is positive electrode while a carbon material is negative electrode, by using a specified mixed solvent as an organic solvent of the electrolyte.

CONSTITUTION: LixMo2 (where M means at least one species among Co, Ni, or Mn, while (x) is 0.5≤x≤1) are used for a positive electrode 2, while a carbonaceous material obtained by carbonizing a petroleum pitch through baking and the like, is used for a negative electrode 1. An organic solvent of an electrolyte is a mixed solvent of propylene carbonate and ester compound, which is expressed as R1COOR2 (where R1, R2 are alkyl group chosen out of CH3, C2H5, C3H7 and C4H9. Charging/discharging cycle of this compound is good even under a high temperature atmosphere, because of the high boiling point of the compound and so on, while the energy density of the compound is high and is thus polution-free.