A positive electrode active material for a nonaqueous electrolyte secondary battery includes at least a lithium-containing manganese layered composite oxide represented by the general formula Li1-xMO2-y-.delta.Fy. The second metallic element or constituent M may be Mn or a combination of Mn and substitute metal such as Co, Ni, Cr, Fe, Al, Ga or In. A lithium deficiency quantity x is in the range of 0