Positive electrode active material and nonaqueous electrolyte secondary battery

The present invention provides a positive electrode (2) active material containing a lithium-containing composite metal oxide having a composition represented by formula (1) given below: LiMgMPO (1) where M is at least one kind of an element selected from the group consisting of Co and Ni, and the molar ratio x is larger than 0.5 and smaller than 0.75, i.e., 0.5 < x < 0.75.