A positive electrode active material and a nonaqueous electrolyte secondary battery containing a lithium-containing composite metal oxide having a composition represented by: Li.sub.1+yM.sub.1-y-zM1.sub.zPO.sub.4 where M is at least one of Co and Ni, and M1 is at least one of Mg, Zr and Al, the molar ration y is larger than 0 and smaller than 0.5, and the molar ration z is larger than 0 and not larger than 0.5 is provided.