PURPOSE: A method and apparatus for refining molten steel in a converter using residue slag are provided to improve material property of residue slag in order to increase the measurement rate of quicklime. CONSTITUTION: A method for refining molten steel in a converter using residue slag comprises a loading step, a refining step, a step of putting out steel, a fore blow step, and a coating step. Molten iron(20) is inserted into a converter(10) at the loading step. Oxygen flows into the converter to remove foreign matters at the refining step. The refined molten steel moves to a ladle(27). Residue slag reacts with oxygen by positioning an oxygen supply lens in a converter after the step of putting out steel. In the coating step, the residue slag is coated on the inner wall of the converter after the fore blow step.