A lithium-containing complex oxide exhibits a high performance as a cathode active material of a lithium secondary cell or the like and having a high tap density. A granular lithium-containing complex oxide, such as lithium manganese complex oxide, is made up of "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. A metal oxide such as manganese oxide and lithium carbonate not more than 5 .mu.m in average grain size are mixed by means of a mixer which grinds and mixes particles by using a shearing force and heated and fired at a warming rate of not more than 50.degree. C./h., thus producing the lithium-containing complex oxide.