NOVELTY - A mixed solution containing lithium compound, manganese compound, iron compound, phosphorus compound and sugar or organic acid is neutralized, to obtain aqueous slurry having pH of 5.5-12.5. The slurry is heat-treated at 120-220 degrees C and washed, to obtain lithium-manganese-iron compound (I). An organic substance is added to the obtained compound, to obtain precursor powder which is then baked at 250-850 degrees C by inert gas or reducing gas atmosphere having oxygen content of 0.1% or less, to obtain olivine-structured lithium manganese iron phosphate particulate powder.    USE - Production of olivine-structured lithium manganese iron phosphate particulate powder used as positive electrode active material for nonaqueous electrolyte secondary battery (claimed) for personal computer, electric tool and electric vehicle.    ADVANTAGE - The method efficiently and economically provides olivine-structured lithium manganese iron phosphate particulate powder particulate powder. The battery using the powder has excellent charging/discharging capacity, electric current load characteristics, and energy density.    DETAILED DESCRIPTION - A mixed solution containing lithium compound, manganese compound, iron compound, phosphorus compound and sugar or organic acid is neutralized, to obtain aqueous slurry having pH of 5.5-12.5. The slurry is heat-treated at 120-220 degrees C and washed, to obtain lithium-manganese-iron compound (LiMn1-xFexPO4) (I) (where x is 0.05-0.5) (75 wt.% or more) having crystal grain size of 10-250 nm. An organic substance (3-40 wt.%) is added to the obtained compound, to obtain precursor powder which is then baked at 250-850 degrees C by inert gas or reducing gas atmosphere having oxygen content of 0.1% or less, to obtain olivine-structured lithium manganese iron phosphate of formula: LiMn1-xFexPO4 (where x is 0.05-0.5) particulate powder. The molar ratio of content of lithium to sum of content of manganese and phosphorus is 0.95-2. The molar ratio of content of phosphorus to sum of content of manganese and iron is 0.95-1.3. The molar ratio of sugar or organic acid to sum of content of manganese and iron is 1-20.