Proposed is a method for producing a composite oxide containing lithium and manganese, whereby it becomes possible to produce a quality-stable composite oxide containing lithium and manganese even when the sodium content in manganese dioxide is high in some degree and it also becomes possible to reduce the strain of the composite oxide containing lithium and manganese to a low level. Proposed is a method for producing a composite oxide containing lithium and manganese, said method being characterized by comprising: mixing manganese dioxide which is a manganese raw material containing a sulfur (S) element and also containing a sodium (Na) element in an amount of 0.1 mass% or more, a lithium raw material and other metal raw material together to prepare a raw material mixture composition and, at the same time, adjusting the ratio of the content of S to the content of Na (i.e., S/Na) in the raw material mixture composition to 0.4 or more by mole; and subsequently burning the raw material mixture composition at 500 C or higher.