A positive electrode active material for lithium ion batteries, which is composed of at least lithium and transition metal, the positive electrode active material being a lithium-containing transition metal oxide, and the transition metal in the lithium-containing transition metal oxide being at least one or two or more kinds selected from the group consisting of Ni, Mn, Co and Fe, wherein the positive electrode active material comprises secondary particles thereof, wherein a variation in the composition ratio of transition metal, relative to all metals, present in the positive electrode active material inside the secondary particles or between the secondary particles of the positive electrode active material is 5% or less, wherein the said variation in composition ratio of transition metal is defined as a numerical ratio of: (a) the absolute value of the difference between (i) a composition ratio of transition metal, relative to all metals, present inside the secondary particles or between the secondary particles, which composition ratio of the or each respective transition metal is measured by a field-emission electron probe microanalyzer (FE-EPMA) at 3 positions inside the secondary particles and 3 positions between the secondary particles, and the contents of each respective transition metal is calculated in terms of moles and then recalculated as a composition ratio, and then each respective transition metal's composition ratio is expressed as the ratio of a molar amount of the respective transition metal and a total molar amount of all metals, and (ii) a composition ratio of transition metal, relative to all metals, in the positive electrode active material in a bulk state as measured by ICP-OES, to (b) the said composition ratio of transition metal, relative to all metals, in the positive electrode active material in the bulk state. wherein a variation in the composition ratio of transition metal, relative to all metals, present in the positive electrode active material inside the secondary particles or between the secondary particles of the positive electrode active material is 5% or less, wherein the said variation in composition ratio of transition metal is defined as a numerical ratio of: (a) the absolute value of the difference between (i) a composition ratio of transition metal, relative to all metals, present inside the secondary particles or between the secondary particles, which composition ratio of the or each respective transition metal is measured by a field-emission electron probe microanalyzer (FE-EPMA) at 3 positions inside the secondary particles and 3 positions between the secondary particles, and the contents of each respective transition metal is calculated in terms of moles and then recalculated as a composition ratio, and then each respective transition metal's composition ratio is expressed as the ratio of a molar amount of the respective transition metal and a total molar amount of all metals, and (ii) a composition ratio of transition metal, relative to all metals, in the positive electrode active material in a bulk state as measured by ICP-OES, to (b) the said composition ratio of transition metal, relative to all metals, in the positive electrode active material in the bulk state. (a) the absolute value of the difference between (i) a composition ratio of transition metal, relative to all metals, present inside the secondary particles or between the secondary particles, which composition ratio of the or each respective transition metal is measured by a field-emission electron probe microanalyzer (FE-EPMA) at 3 positions inside the secondary particles and 3 positions between the secondary particles, and the contents of each respective transition metal is calculated in terms of moles and then recalculated as a composition ratio, and then each respective transition metal's composition ratio is expressed as the ratio of a molar amount of the respective transition metal and a total molar amount of all metals, and (ii) a composition ratio of transition metal, relative to all metals, in the positive electrode active material in a bulk state as measured by ICP-OES, to (i) a composition ratio of transition metal, relative to all metals, present inside the secondary particles or between the secondary particles, which composition ratio of the or each respective transition metal is measured by a field-emission electron probe microanalyzer (FE-EPMA) at 3 positions inside the secondary particles and 3 positions between the secondary particles, and the contents of each respective transition metal is calculated in terms of moles and then recalculated as a composition ratio, and then each respective transition metal's composition ratio is expressed as the ratio of a molar amount of the respective transition metal and a total molar amount of all metals, and (ii) a composition ratio of transition metal, relative to all metals, in the positive electrode active material in a bulk state as measured by ICP-OES, to (b) the said composition ratio of transition metal, relative to all metals, in the positive electrode active material in the bulk state.