A drainage treatment system 16 of the invention is a drainage treatment system that treats drainage generated when a gasified gas 33 is produced by a coal gasification furnace 12 and is purified by a gas purification device 14, and includes drainage treatment lines L11 to L15 which respectively treat slag drainage, venturi drainage, and stripper drainage generated when the gasified gas 33 is produced and the gasified gas 33 is cleaned and drainage treatment apparatuses 101A to 101E which treat treatment target materials in the drainages discharged from the drainage treatment lines L11 to L15. Accordingly, the drainages of the drainage treatment lines L11 to L15 are respectively and individually treated in response to the treatment target materials contained in the drainages without mixing the drainages of the drainage treatment lines L11 to L15.