[Object] Provided is a means which is capable, with respect to a non-aqueous electrolyte secondary battery, of suppressing a decrease in capacity when the battery is used for a long period of time, and improving cycle characteristics.[Solving Means] Provided is a positive electrode active substance for a non-aqueous electrolyte secondary battery, the positive electrode active substance being a lithium-nickel-manganese-cobalt composite oxide and having true density of 4.40 to 4.80 g/cm3.