A main object of the present invention is to provide a lithium secondary battery whose potential can be rapidly increased immediately after the start of overcharge, thereby terminating charging before a safety valve is actuated. This object can be solved by a lithium secondary battery comprising: a cathode having a phosphate compound of olivine structure as a cathode active material, an anode, a separator sandwiched between the cathode and the anode, an electrolyte comprising a supporting salt dissolved therein, and a safety valve having a valve-opening pressure within the range of 25 kgf/cm2 to 30 kgf/cm2, wherein assuming that the initial amount of Li contained in the cathode is 100, the initial amount of Li contained in the electrolyte is within the range of 5 to 20.