A molten steel refining apparatus and a method therefor, in which the carbon component of molten steel can be easily removed, the temperature drop of molten steel can be effectively reduced, and a stable operation is realized. The apparatus for refining molten steel for manufacturing ultra low carbon steel includes an RH vacuum-degassing device consisting of a vessel and a snorkel composed of an up-leg and a down-leg. The apparatus further includes a plurality of gas injection lance nozzles each consisting of an inner tube and an outer tube, and installed on the side wall of the vessel of the RH vacuum-degassing device so as to inject gas toward molten steel within the vessel. The inner tube includes a throat for forming a jet stream of super-sonic velocity, and the outer tube injects cooling gas for cooling the inner tube.