Provided is a “lithium-excess-type” active material in which a decrease in potential associated with a charge-discharge cycle is suppressed. Disclosed is a positive active material for a nonaqueous electrolyte secondary battery containing a lithium transition metal composite oxide. In this positive active material, the lithium transition metal composite oxide has an α-NaFeO2 structure, a molar ratio Li/Me of Li to a transition metal (Me) is 1.25≤Li/Me≤1.45, Ni and Mn are contained as the transition metal (Me), an X-ray diffraction pattern attributable to a space group R3-m is included, and a half-value width for a diffraction peak of a (104) plane at a Miller index hkl in X-ray diffraction measurement using a CuKα ray is 0.27° or less.