A main object is to provide a method of producing a cathode active material having a high average discharge potential, and a high degree of stability at high potential. A step of preparing a Na-doped precursor of making a transition metal oxide having a P2 structure belonging to a space group of P63/mmc; and an ion exchange step of substituting lithium for at least part of sodium contained in the transition metal oxide by an ion exchange method are included, wherein the transition metal oxide has the composition represented by NaaMnx−pNiy−qCoz−rM1p+q+rO2, where 0.5≤a≤1, x+y+z=1, 3≤4x+2y+3z≤3.5, and 0.05≤p+q+r