NOVELTY - A lithium manganese system complex oxide comprises complex oxide containing a monoclinic crystal layered rock salt-type structure (I). The molar ratio of manganese, iron and nickel in the triangular composition diagram corresponds to point A (60:40:0), point B (40:60:0) and point C (70:0:30), and makes a tetragon with point D (80:0:20). The average oxidation number of manganese, iron and nickel is 3.4-3.6.    USE - Lithium manganese system complex oxide is used as positive electrode material for lithium ion secondary battery (all claimed) used in mobile phones, notebook personal computer, electric vehicles and electric power load-leveling system.    ADVANTAGE - The lithium manganese system complex oxide has large surface area and has reduced generation of ferrite compound, which does not contribute to charging and discharging reactions. The lithium manganese system complex oxide provides positive electrode material with excellent charge and discharge capacitance, and lithium ion secondary battery with excellent performance.    DETAILED DESCRIPTION - A lithium manganese system complex oxide comprises complex oxide containing a monoclinic crystal layered rock salt-type structure of formula: Li1+x(Mn1-m-nFemNin)1-xO2 (I). The molar ratio of manganese, iron and nickel in the triangular composition diagram corresponds to point A (60:40:0), point B (40:60:0) and point C (70:0:30), and makes a tetragon with point D (80:0:20). The average oxidation number of manganese, iron and nickel are 3.4-3.6.    x=0-1/3;    m=0-0.6;and    n=0-0.3.    An INDEPENDENT CLAIM is included for manufacture of lithium manganese system complex oxide, which involves mixing aqueous solution containing manganese, iron and nickel compound to form precipitate, adding lithium compound to the formed precipitate, carrying out wet-oxidation process and baking the resultant product in oxidative atmosphere and then baking in inert gas atmosphere.    DESCRIPTION OF DRAWING(S) - The drawing shows a triangular composition diagram, which shows molar ratio of transition metal element contained in lithium manganese system complex oxide. (Drawing includes non-English language text)