An embodiment of the present invention provides a method to reuse slag wherein molten steel is outputted from a converter; calcium oxide in remaining slag is stabilized by injecting silicon dioxide to the converter; and the stabilized slag is cooled and be reused. The present invention provides the method to reuse slag comprising: a step of outputting molten steel in a converter wherein molten steel and slag are produced therein; a step of injecting silicon dioxide (SiO_2) to the converter to react to calcium oxide in order to stabilize the calcium dioxide in the slag remaining in the converter after outputting the molten steel; and a step of cooling the slag to reuse the slag.
