The present invention provides a positive electrode active material for a non-aqueous electrolyte-based secondary battery, composed of a lithium/nickel composite oxide with high capacity, low cost and excellent heat stability, and a high safety non-aqueous electrolyte-based secondary battery. A positive electrode active material, comprising lithium/nickel composite oxide powders obtained by water washing fired powders having the following composition formula (1), followed by filtering and drying: LiNi1-aMaO2   (1) (wherein, M represents at least one kind of an element selected from transition metal elements other than Ni, group 2 elements, or group 13 elements; and “a” satisfies 0.01≦a≦0.5), characterized in that specific surface area of the lithium/nickel composite oxide powders after water washing is 0.3 to 2.0 m2/g.