NOVELTY - A lithium secondary battery, comprises a positive electrode (1); a negative electrode (2); and a nonaqueous electrolytic solution containing a lithium salt, where there is an state of charge (SOC) region having a specific input power is almost equal to a specific output power of 20-40% of the SOC of the lithium secondary battery.    USE - As a lithium secondary battery for a plug-in hybrid electric vehicle (claimed), in particular, but is widely applicable to other uses, for example, to the fields of a fuel cell electric vehicle, an electric vehicle, and a power tool.    ADVANTAGE - The lithium secondary battery has excellent input and output balance, a high capacity, and a long life. The lithium secondary battery have a well balanced input and output characteristic. Since the active material quantity has been optimized, a lightweight and compact lithium secondary battery having a well-balanced specific input power and specific output power in a low SOC region can be obtained.    DETAILED DESCRIPTION - A lithium secondary battery, comprises a positive electrode (1) containing a positive electrode active material which is applied to both sides of a positive electrode collector foil and contains a lithium transition metal complex oxide; a negative electrode (2) containing a negative electrode active material which is applied to both sides of a negative electrode collector foil and occludes and releases lithium; and a nonaqueous electrolytic solution containing a lithium salt, where there is an state of charge (SOC) region having a specific input power is almost equal to a specific output power of 20-40% of the SOC of the lithium secondary battery. In the lithium secondary battery, a ratio between a quantity (Mc) (mg/cm2) of the positive electrode active material per unit area in one side of the positive electrode collector foil and a quantity (Ma) (mg/cm2) of the negative electrode active material per unit area in one side of the negative electrode collector foil is 0.5-0.7. In the lithium secondary battery, quantity Mc (mg/cm2) of the positive electrode active material per unit area in one side of the positive electrode collector foil is 9-14 mg/cm2. The secondary battery has a specific energy of not less than 100 W/kg.    DESCRIPTION OF DRAWING(S) - The figure shows a side cross-sectional view of the lithium secondary battery.    Positive electrode (1)    Negative electrode (2)    Separator (3)    Battery can (4)    Positive electrode collector lead piece (5)    Negative electrode collector lead piece (6)