To provide a gasification melting furnace in which attachment and growth of ash on inner parts of a diffuser can be prevented.

This gasification melting furnace comprises the diffuser 11 to provide waste in the form of ash from a gasification furnace, a melting furnace main body 31 to melt noncombustible matter in the waste provided from the diffuser 11, and a slag discharge means 41 to discharge ash in the form of slag from the melting furnace main body 31. In this gasification melting furnace, the diffuser 11 is composed in a restriction structure 15 having a throat in the middle. A boiler 51 is disposed around piping to provide waste from the gasification furnace to the diffuser, and above the diffuser 11. A steam pipe is disposed in a wall of the diffuser 11, so that the diffuser 11 is cooled by steam supplied from the boiler 51 to the steam pipe.