The present invention provides a positive electrode active material. The positive electrode active material is represented by the following formula (I) and has a BET specific surface area of larger than 5 m2/g and not larger than 15 m2/g: LixM1yM31-yO2  (I) wherein M1 is at least one transition metal element selected from Group 5 elements and Group 6 elements of the Periodic Table, M3 is at least one transition metal element other than M1 and selected from among transition metal elements excluding Fe, x is not less than 0.9 and not more than 1.3, and y is more than 0 and less than 1.