The present invention relates to a method of recovering a valuable metal from slag. The method includes the steps of: applying a magnetic field whose magnitude is increased in a stepwise fashion to crushed slag to separate a magnetic material from the crushed slag; and introducing a reductant into the separated magnetic material to recover a valuable metal therefrom. The method is advantageous and cost effective because the recovery rate of iron (Fe) from slag exceeds 50% by using a magnetic field, thus reducing the amount of slag to be wasted.