PROBLEM TO BE SOLVED : To provide an electrode active material which is high in electron conductivity and which is excellent in diffusivity of Li ions at the inside of a crystal grain or primary grain in the electrode active material.SOLUTION : An electrode active material for a lithium ion secondary battery of the present invention is an electrode active material containing sulfur or a sulfur-containing compound in a crystal grain that comprises a compound represented by general formula LiADPO(where A represents at least one selected from the group consisting of Fe, Mn, Co, and Ni, D represents at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, Y, and a rare earth element, and 0≤a≤1.0, 0
