A system is disclosed provided with: a slag bath that retains slag cooling water and receives slag; a slag cooling water feed line and a slag pump outlet via which the slag and the slag cooling water are pumped and fed to a slag separation device; a circulation pump that forms a water flow for pumping out the slag; a slag cooling water circulation line linking from the slag separation device to the slag bath; a reverse flow line that causes the slag cooling water dispensed from the circulation pump to flow in reverse in the slag cooling water feed line; a selective supply unit capable of selectively supplying the slag cooling water dispensed from the circulation pump to the slag cooling water circulation line and the reverse flow line; and a water supply line, one end of which is connected to the slag cooling water feed line.