In a treatment process for a positive electrode active material for a lithium-ion secondary battery, Li compounds such as Li2CO3, LiOH, and the like, present in a positive electrode active material for a lithium-ion secondary battery but unsuitable for a positive electrode material, is removed. In a case wherein the resultant Li compound is used as a positive electrode material of the secondary battery, a discharge capacity and an average discharge voltage of the secondary battery do not decline, and gelation caused by a cleaning treatment is prevented. The positive electrode active material is cleaned with a cleaning fluid containing NH3, and then solid-liquid separation is performed, and a solid component is calcined at 600 to 700° C. in an oxygen atmosphere. The cleaning fluid has a conductivity of 11.6 mS/cm or less, and contains a recovered liquid component and may be repeatedly used as a cleaning fluid.