A positive electrode active material containing lithium manganate having a spinel structure is used, wherein the value of a* of the L*a*b* color specification system is made to satisfy the requirement: -3.2 .ltoreq.a*.ltoreq.-1.4. Or, the value of a* is made to satisfy the requirement: -5.8 23 a*.ltoreq.-4.2. Or, the value of L* is made to satisfy the requirement: 24 .ltoreq.L*.ltoreq.27.