Disclosed are a positive electrode active material and a method for producing an olivine-type phosphate. The positive electrode active material comprises an olivine-type phosphate represented by the following formula (I), wherein the maximum peak in an X-ray diffraction pattern obtained using a CuKα ray is the peak of the (031) plane of the olivine-type phosphate and the half-value width of the peak is 1.5° or less: AaMbPO4 (I), wherein A represents one or more elements selected from among alkali metals; M represents one or more elements selected from among transition metals; a is from 0.5 to 1.5; and b is from 0.5 to 1.5. The method for producing an olivine-type phosphate comprises preparing a raw material comprising element A, element M, and phosphorus (P) so that a A:M:P molar ratio may be a:b:1, preliminary calcining the raw material, and mainly calcining the preliminary calcined raw material.