NOVELTY - Manufacture of sodium-containing transition metal oxide involves mixing compound containing 3-5 atomic% transition metal element (Me') and baking the mixture to produce sodium-containing transition metal oxide. The sodium-containing transition metal oxide (I) has crystal structure of layered P2-type or layered O3-type.    USE - Manufacture of sodium-containing transition metal oxide used for manufacture of positive electrode (claimed) for sodium ion secondary battery e.g. sodium molten salt battery. Uses include but are not limited to domestic electric power storage apparatus, industrial electric power storage apparatus, electric vehicle and hybrid vehicle.    ADVANTAGE - The method enables efficient manufacture of sodium-containing transition metal oxide and manufacture of positive electrode without gelatinization of the positive mix slurry. The sodium-containing transition metal oxide provides sodium ion secondary battery having high capacitance.    DETAILED DESCRIPTION - Manufacture of sodium-containing transition metal oxide involves mixing compound containing 3-5 atomic% transition metal element (Me') and baking the mixture to produce sodium-containing transition metal oxide. The sodium-containing transition metal oxide has crystal structure of layered P2-type or layered O3-type. The sodium-containing transition metal oxide is of formula: NaxMe'O2 (I). When the stoichiometric ratio of sodium with respect to the element (Me') is xq, during mixing of compound containing element (Me'), the ratio of 3-5 atomic% of sodium with respect to element (Me') is less than xq.    Me'=transition metal element chosen from Ni, Ti, Mn, Fe and Co, and Ni and/or Fe.    An INDEPENDENT CLAIM is included for manufacture of positive electrode (2), which involves using mixture of sodium-containing transition metal oxide, conductive support agent, binder and dispersion medium.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of sodium-ion secondary battery.    Separator (1)    Positive electrode (2)    Negative electrode (3)    Gasket (9)    Battery case (10)