An aqueous niobium oxalate solution is prepared by dissolving a niobic acid in an aqueous oxalic acid solution. The oxalic acid/niobium molar ratio in the aqueous niobium oxalate solution is generally in the range of from 1 to 10, preferably from 2 to 6, most preferably from 2 to 4. To the obtained aqueous niobium oxalate solution is added aqueous hydrogen peroxide to obtain an aqueous niobium-hydrogen peroxide solution. The hydrogen peroxide/niobium molar ratio in the aqueous niobium-hydrogen peroxide solution is preferably in the range of from 0.2 to 10, more preferably from 0.4 to 8, most preferably from 2 to 6. It is preferred that the temperature of the obtained aqueous niobium-hydrogen peroxide solution is maintained at 65° C. or less, more advantageously 50° C. or less. The niobium concentration of the aqueous niobium-hydrogen peroxide solution is preferably 0.05 mol/kg or more, more preferably 0.15 mol/kg or more.
