Deoxidation of solid niobium using metallic liquid calcium or magnesium was studied. In the temperature range of 973K to 1473K, the equilibrium oxygen concentrations under the coexistence of the reductant and its oxide were analyzed. An oxygen level of a few tens of mass ppm was found when Ca and CaO coexisted, and a level of an about 100 ppm when Mg and MgO coexisted. Ca deoxidation could be applied under an inert gas atmosphere even at temperatures about 200K lower than those applied for the Ti external gettering. The thermodynamic data reported in the binary system of Nb-O in the high oxygen concentration range or at higher temperatures could not explain the measured oxygen solubilities in Nb under the coexistence of Ca-CaO or Mg-MgO.