A slag discharge system includes a slag bath at a bottom portion of a gasifier; a slag cooling water circulation line that discharges a mixture of slag and a slag cooling water from the slag bath; a coarse slag separator device that separates coarse slag contained in the slag cooling water; a fine slag separator device connected to the coarse slag separator device downstream side, the fine slag separator device separating fine slag; and a circulator pump connected to the fine slag separator device downstream side, the circulator pump creating a water flow in the slag cooling water circulation line; wherein the fine slag separator device includes a branch section where the slag cooling water circulation line branches into a plurality of branch lines and again joins together as one line, a fine slag filter portion, and a shutoff valve and disposed in each of the plurality of branch lines.