To provide an active substance for a nonaqueous electrolyte secondary cell having a large discharge capacity and excellent high discharge performance.Provided is an active substance for a nonaqueous electrolyte secondary cell that contains a lithium transition metal composite oxide that has an alpha-NaFeO2-type crystal structure and is represented by the composition formula Li1+alphaMe1-alphaO2 (where Me is a transition metal element such as Co, Ni, or Mn and alpha is greater than 0), and a method for producing the same. The lithium transition metal composite oxide is characterized by the following: having a molar ratio of Li to a transition metal element Me of 1.2 to 1.4; a D10 of 6 to 9 [mu]m, D50 of 13 to 16 [mu]m, and D90 of 18 to 32 [mu]m where the grain size at which the integrated volume in the grain size distribution of secondary particles is 10%, 50%, and 90% is represented by D10, D50, and D90; and the particle diameter of primary particles is 1 [mu]m or smaller.