PURPOSE: A device and method for treating slag are provided to perform efficient high-speed processing and taking off process of slag by the temperature control and circulation of the coagulated slag. CONSTITUTION: A device for treating slag comprises a slag quenching part(10), a coagulated slag collecting part, a by-product process part(90) and a coagulated slag discharge part(60). The by-product process part processes a by-product including hot gas generated in the quenching of slag. The by-product process part is connected to one of the slag quenching part and coagulated slag collecting part. The slag quenching part comprises a unit body(14), a nozzle member(16) and a slag runner(18). The unit body is installed at an input path of melting slag. The nozzle member emits gas to the unit body and adjusts one of height and injecting angle.