By means of a series of wet multistage oxidation process comprising: Step 1 for adding an alkaline reagent to an aqueous solution of a manganese compound containing a divalent manganese thereby precipitating a manganese hydroxide; Step 2 for adding an aqueous hydrogen peroxide while keeping the temperature of the water of the aqueous solution comprising the manganese hydroxide at room temperature thereby converting into a manganese oxide; and also Step 3 for adding a dilute acid to the manganese oxide in a state where the water is coexisting, a nanometer-sized manganese dioxide having a ramsdellite-type crystal structure is obtained.