A method is employed for producing a positive electrode active material for a lithium secondary battery that comprises mixing lithium phosphate having a particle diameter D90 of 100 μm or less, an M element-containing compound having a particle diameter D90 of 100 μm or less (where, M is one type or two or more types of elements selected from the group consisting of Mg, Ca, Fe, Mn, Ni, Co, Zn, Ge, Cu, Cr, Ti, Sr, Ba, Sc, Y, Al, Ga, In, Si, B and rare earth elements) and water, adjusting the concentration of the M element with respect to water to 4 moles/L or more to obtain a raw material, and producing olivine-type LiMPO4 by carrying out hydrothermal synthesis using the raw material.