NOVELTY - A positive electrode active material (T1) comprises positive electrode active material particles composed of lithium transition metal complex oxide having layered structure containing nickel and manganese as main components; and at least one niobium-containing material selected from lithium-niobium-oxygen (Li-Nb-O) compound and lithium-nickel-niobium-oxygen (Li-Ni-Nb-O) compound, where the niobium-containing material is sintered onto surfaces of the positive electrode active material particles and forms a solid solution portion with the positive electrode active material particles.    USE - As a positive electrode active material used in non-aqueous electrolyte secondary battery (claimed); and as a power supply for hybrid vehicles.    ADVANTAGE - The positive electrode active material reduces the weight and increases the capacity of nonaqueous electrolyte secondary batteries; and prevents short-circuiting caused by the contact between the positive and negative electrodes, thereby yielding lithium ion conductivity. The positive electrode active material includes an inexpensive lithium transition metal complex oxide having a layered structure containing nickel and manganese as main components, such that the output characteristics under various temperature conditions and after high-temperature storage can be improved.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing the positive electrode active material (T1) involving obtaining positive electrode active material particles composed of a lithium transition metal complex oxide having a layered structure containing at least nickel and manganese by primary baking, and subjecting a mixture of the positive electrode active material particles and a niobium-containing material to secondary baking at a temperature lower than that of the primary baking to form at least one niobium-containing material selected from Li-Nb-O compound and Li-Ni-Nb-O compound sintered onto surfaces of the positive electrode active material particles; and    (2) a nonaqueous electrolyte secondary battery comprising a positive electrode including the positive electrode active material (T1), a negative electrode including a negative electrode active material, and a nonaqueous electrolyte prepared by dissolving a solute in a nonaqueous solvent.