A high-capacity positive electrode active material is provided that enables a non-aqueous electrolyte secondary battery to have excellent load characteristics and high initial charge-discharge efficiency. A non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a non-aqueous electrolyte, and performs charge and discharge by transferring lithium ions between the positive electrode and the negative electrode. The positive electrode has a positive electrode mixture and a positive electrode current collector. The positive electrode mixture contains a positive electrode active material, a conductive agent, and a binder agent (binder). The positive electrode active material includes Li1+x−a(MnyM1−y)1−xO2±b, where 0