A positive electrode active material and a nonaqueous electrolyte secondary battery containing a lithium-containing composite metal oxide having a composition represented by: Li_1+yM_1-y-zM1_zPO₄ where M is at least one of Co and Ni, and M1 is at least one of Mg, Zr and Al, the molar ratio y is larger than 0 and smaller than 0.5, and the molar ratio z is larger than 0 and not larger than 0.5 is provided.