An active material for a non-aqueous electrolyte secondary battery including a lithium-containing transition metal oxide containing nickel and manganese and having a closest-packed structure of oxygen, wherein an atomic ratio MLi/MT between the number of moles of lithium MLi and the number of moles of transition metal Mt contained in the lithium-containing transition metal oxide is greater than 1.0; the lithium-containing transition metal oxide has a crystal structure attributed to a hexagonal system, and the X-ray diffraction image of the crystal structure has a peak P003 attributed to the (003) plane and a peak P104 attributed to the (104) plane; an integrated intensity ratio I003/I104 between the peak P003 and the peak P104 varies reversibly within a range from 0.7 to 1.5 in association with absorption and desorption of lithium by the lithium-containing transition metal oxide; and the integrated intensity ratio varies linearly and continuously.