To provide a slag solidification system having excellent treating ability in a small space.

The slag solidification system for solidifying slag discharged from an electric furnace 2 comprises a tilting device 6 tilting the electric furnace 2 to discharge slag from a slag discharge port 4 of the electric furnace 2; a plurality of molds 8 each having a bottom wall and a side wall; a slag carrying-discharging device carrying out a carry-in process for moving the empty mold 8 out of the molds 8 into a slag discharge position below the slag discharge port 4, a carry-out process for moving the mold 8 which has received the slag from the slag discharge position to a cooling position, and a slag discharge process for discharging solidified slag from the mold 8 in the cooling position; and a control device for controlling the tilting device 6 to regulate the thickness of slag in the mold 8 in a range from 10 mm to 300 mm.