The present invention provides a method for producing a positive electrode active material for lithium ion battery, having excellent tap density, at excellent production efficiency, and a positive electrode active material for lithium ion battery. The method for producing a positive electrode active material for lithium ion battery including a step of conducting a main firing after increasing mass percent of all metals in lithium-containing carbonate by 1% to 105% compared to the mass percent of all metals before a preliminary firing, by conducting the step of a preliminary firing to the lithium-containing carbonate, which is a precursor for positive electrode active material for lithium ion battery, with a rotary kiln.