To efficiently melt ash by blowing flame containing powder such as ash directly into molten ash.

Assist fuel and oxidizer are fed from injection nozzles 3a to 3c into a melting tank 2, material ash to be melted is introduced, and burnt to be melted. The material ash is first melted in a flame. When molten ash is stored over mounting positions of injection nozzles 3a to 3c in the tank 2, fuel and oxidizer as well as material ash to be melted are directly blown into the molten ash this time, passed to rise in the molten ash as bubble together with combustion gas, and melted in this process.