The present method is to produce an active material powder formed of a spinel oxide containing lithium or a layer-structured oxide containing lithium for a lithium secondary battery which is uniform in composition, fine in particle size and free of oxygen defects, and which is unlikely to cause capacity deterioration resulted from repetitive charge/discharge cycles at a high current density.A suspension 1 prepared by suspending an ingredient of the active material powder in a combustible liquid or an emulsion prepared by emulsifying a solution of the ingredient in the combustible liquid is sprayed in a droplet state 15 together with an oxygenic gas 2. The combustible liquid contained in the droplet 15 is burned to have the ingredient therein reacted and to evaporate the solvent. As a result, active material powder 4 formed of the spinel oxide containing lithium is obtained. An active material powder formed of the layer-structured oxide containing lithium is obtained by re-heating the oxide powder that has been generated by spraying and burning the droplet.