Means for stably supplying a lithium-containing complex oxide exhibiting a high performance as a cathode active material of a lithium secondary cell or the like and having a high tap density. Granular lithium-containing complex oxide such as lithium manganese complex oxide comprises “complex oxide grains produced by integrating lithium-rich material grains abnormally grown during a firing reaction with the surfaces of the base grains by sintering.” The number of complex oxide grains is not more than 50 per gram of the complex grains. Metal oxide such as manganese oxide and lithium carbonate not more than 5 μm in average grain size are mixed by means of a mixer which grinds and mixes particles by using shearing force and heating and firing them at a warming rate of not more than 50° C./h., thus producing the lithium-containing complex oxide.