PURPOSE: To manufacture lithium-containing manganese composite oxide excellent in a cycle characteristic by mixing heat-treated electrolytic manganese dioxide with a lithium compound at a specific ratio, and adding nitric acid thereinto, followed by a specific heat treatment.

CONSTITUTION: Electrolytic manganese dioxide or chemosynthetic manganese dioxide is subjected to a heat treatment at about 250-650°C so as to remove the water content. The product resulting from the heat treatment is mixed with a lithium compound such as LiNO3 in the atomic ratio of 0.45≤Li/Mn≤0.55. The resultant mixture is added with HNO3, followed by a heat treatment, where it is heated once at 500°C or lower, and subsequently, at 500°C or higher. Consequently, it is possible to synthesize lithium-containing manganese composite oxide of spinel crystal structure at a low cost. Use of the lithium-containing manganese composite oxide as a positive electrode active material can improve a cycle characteristic of a non-aqueous electrolyte secondary battery where a carbon material is used as a negative electrode active material.