NOVELTY - A positive electrode active material comprises lithium transition metal oxide (I) and oxoacid salt. The positive electrode active material shows a peak (A) derived from phosphorus and a peak (B) derived from transition metal, when detected by transmission electron microscope energy dispersive X-ray analysis. The ratio of intensity of peak (A) having depth of 50 nm from the positive electrode active material surface at 50 nm with respect to intensity of peak (A) having depth of 5 nm from the positive electrode active material surface at 5 nm is 0.00001-0.5.    USE - Active material is used for positive electrode of nonaqueous electrolyte secondary battery (all claimed) used as power supply for consumer appliances and motor vehicles e.g. hybrid vehicle and electric vehicle.    ADVANTAGE - The positive electrode active material provides secondary battery with excellent cycle characteristics, storage characteristics, and continuous charge and discharge characteristics even at high temperature.    DETAILED DESCRIPTION - A positive electrode active material comprises lithium transition metal oxide of formula: Li1+aMn1.5-bNi0.5-cQdO4-(~d) (I) and oxoacid salt. The average operating electric potential of lithium transition metal oxide is 4.5 V or more with respect to lithium standard. The positive electrode active material shows a peak (A) derived from phosphorus and a peak (B) derived from transition metal, when detected by transmission electron microscope energy dispersive X-ray analysis in the cross-section of positive electrode active material. The ratio of intensity of peak (A) having depth of 50 nm from the positive electrode active material surface at 50 nm with respect to intensity of peak (A) having depth of 5 nm from the positive electrode active material surface at 5 nm is 0.00001-0.5. The ratio of intensity of peak (B) having depth of 50 nm from the positive electrode active material surface at 50 nm with respect to intensity of peak (B) having depth of 5 nm from the positive electrode active material surface at 5 nm is 2-10000.    a,b,c=-0.3-0.3;    d=0-0.3;    delta =-0.2-0.2;and    Q=element chosen from Ni, transition metals other than Mn and Na, K, Mg, Ca, Sr, Ba, Al, Ga, In, Si, Ge, Sn, P, Sb, B, or S.    An INDEPENDENT CLAIM is included for nonaqueous electrolyte secondary battery.