To provide a secondary battery indicating a good discharge characteristic when lithium manganese oxide having a spinel structure is used for a positive electrode, by containing a specific amount of malonic diester in organic electrolyte.

Organic electrolyte contains 0.5-3.0 vol% of malonic diester. Preferably, the malonic diester is diethyl malonate or di-n-propyl malonate. Since the organic electrolyte contains the malonic diester, a cycle characteristic is greatly improved. The malonic diester decomposes on a negative electrode surface during charge, good coating for accelerating going in/out of lithium ions is formed. As solvent of 97-99.5% other than the malonic diester, for example, cyclic carbonate class such as ethylene carbonate, chain carbonate class such as dimethyl carbonate or diethyl carbonate, lactone class such as γ- buthrolactone, or composite solvent such as sulfolane is used.