PURPOSE: A Nb solution acidified with oxalic acid is allowed to react with Mg and/or Mn and the resultant solution is thermally decomposed to give the titled fine particles of uniform size which has high purity, good dispersibility, uniform composition and reactivity.

CONSTITUTION: A precipitate of niobium pentahydroxide and a powder of oxalic acid are mixed and dissolved in water and the resultant solution is allowed to react with Mg and/or Mn of 0.1W1mm particle sizes. Then, the reaction mixture is made into fine drops by spraying or atomizing and subjected to thermal decomposition in a short time in a reactor such as an electric furnace at 600W1,300°C, further roasted, if the thermal decomposition is incomplete, to give the title fine powder.