To remove slag deposited on a liquid slag discharge port of a gasification furnace in a reactor for quick thermal decomposable of coal without affecting a thermal decomposition furnace of coal.

This apparatus for thermal decomposition of coal is equipped with a slag discharge port at the bottom of a gasification furnace, has one or more feed openings of gasification burners in the gasification furnace and is furnished with one or more steam feed openings higher than the feed openings of gasification burners. This method for thermal decomposition of coal comprises using an apparatus for thermal decomposition of coal and adjusting the amount of steam fed from the steam feed openings according to the amount of steam fed from the feed openings of the gasification burners of the gasification furnace to raise the temperature at the lower part of the gasification furnace and to remove deposit of the slag discharge port.