Abstract: A positive electrode active material of the present invention comprising a composite oxide containing Li and Ni, and optionally containing at least one element other than Li and Ni, is characterized in one of the following: primary particles constituting each of secondary particles of the composite oxide and having a variation coefficient of span of 17% or less, the span being a formula: (D190?D110)/D150 (D110, D150, D190: particle diameter corresponding to 10%, 50%, 90% of an integrated value in a number standard-particle diameter distribution of primary particle size); the primary particles having a variation coefficient of D150 of 19% or less; and the secondary particles having each of values of 1.00% or less, the values being formulae: |[ER1?ER21)/ER1]|×100, |[ER1?ER22)/ER1]|×100, |[ER1?ER23)/ER1]|×100 (ER1, ER21, ER22, ER23: element ratio (Li/(Ni+Other element(s))) of entire secondary particles, small particles, middle particles, large particles).