Provided is a positive electrode active material which can provide a non-aqueous electrolyte secondary battery having excellent cycle properties. The positive electrode active material of the invention comprises a lithium-containing composite metal oxide having a composition represented by general formula (1): LizFe1-xMxP1-ySiyO4 (wherein M represents at least one metal element selected from Zr, Sn, Y and Al; x represents a numeral value ranging from 0.05 to 1 exclusive; and y represents a numeral value ranging from 0.05 to 1 exclusive), wherein the positive electrode active material has a single crystal phase composed of a lithium-containing composite oxide represented by general formula (1) when z falls within the range from 1 to 0.9-0.75 exclusive and the range from 0.25-0.1 to 0 exclusive, and has co-existing two crystal phases respectively composed of a lithium-containing composite oxide represented by general formula (2): LiaFe1-xMxP1-ySiyO4 (wherein a represents a numeral value ranging from 0.75-0.9 to 1.00 inclusive; x represents a numeral value ranging from 0.05 to 1 exclusive; and y represents a numeral value ranging from 0.05 to 1 exclusive) and a lithium-containing composite oxide represented by general formula (3): LibFe1-xMxP1-ySiyO4 (wherein b represents a numeral value ranging from 0 to 0.1-0.25+x inclusive; x represents a numeral value ranging from 0.05 to 1 exclusive; and y represents a numeral value ranging from 0.05 to 1 exclusive) when z falls within the range from 0.9-0.75 to 0.25-0.1 exclusive, and a value a defined by the formula: a = (bc2)/(bc1) falls within the range from 0.99 to 1.01 exclusive (0.99