NOVELTY - A positive electrode active material comprises lithium-manganese-silver complex oxide formed by substituting at least one portion of tetravalent manganese and element (Me) in lithium-manganese-containing complex oxide (I) by silver. The average particle diameter of primary particle of lithium-manganese-silver complex oxide is 10-200 nm.    USE - Positive electrode active material is used for lithium ion secondary battery (claimed) used as power supply for portable electronic device e.g. notebook personal computer, mobile telephone and electric vehicles.    ADVANTAGE - The lithium ion secondary battery using the positive electrode active material has excellent cycling characteristics.    DETAILED DESCRIPTION - A positive electrode active material comprises lithium-manganese-silver complex oxide formed by substituting at least one portion of tetravalent manganese and element (Me) in lithium-manganese-containing complex oxide of formula: xLi2MnO3.(1-x)LiMeO2 (I) by silver. The average particle diameter of primary particle of lithium-manganese-silver complex oxide is 10-200 nm.    x=more than 0 to 1;and    Me=Nb, Al, Fe, F, Mg, Co, Ti, Ni or trivalent Mn.    INDEPENDENT CLAIMS are included for the following:    (1) lithium ion secondary battery, which has positive electrode containing positive electrode active material, negative electrode and a nonaqueous electrolyte; and    (2) manufacture of lithium-manganese-silver complex oxide, which involves preparing raw material mixture by mixing metallic compound raw material containing metallic compound (A), and oxide and/or hydroxide of silver or metal salt containing metallic element essentially comprising manganese, metallic compound(s) (B) chosen from hydroxide, metal salt, and metal alloy and molten salt raw material containing 50% mass or more of lithium hydroxide, melting the raw material mixture at a temperature which is more than the melting point of the molten salt raw material, and recovering lithium manganese silver complex oxide.