The copper distribution ratio between the Na₂S flux and silver, L_Cu(flux-Ag) (=[mass% Cu]_{(in flux)} / [mass% Cu]_{(in Ag)}), was measured at 1473 K in order to know the copper capacity of Na₂S flux. As the greatest value, L_Cu(flux-Ag) = 42 was obtained. By combining the L_Cu(flux-Ag) value with the distribution ratio of copper between the silver and the carbon-saturated iron, L_Cu(Ag-Fe), the distribution ratio of copper between the Na₂S flux and carbon-saturated iron, L_Cu(flux-Fe), is derived to be 330 at 1473 K. Moreover, the sulfurization removal of copper in iron silver phase into Na₂S flux has been tried at 1473 K. Silver can keep iron from being sulfurized, which enables to maintain the high copper capacity of the Na₂S flux. For this reason, the L_Cu value increases with an increase of sulfur potential. Copper removal proceeds at lower 0.1mass%Cu, and the Cu content decreased to 0.06mass%. The silver phase prevents the sulfur dissolution into the iron, and the sulfur content of iron can be kept lower ([mass%S]_{in Fe-C} < 0.007-0.02).