PURPOSE: To permit perfect melting of ash supplied into a furnace without extinguishing flame produced by fuel by a method wherein a combustion assisting gas supplying means is provided in a furnace body and an exhaust port is formed so as to have a size, making the inside of the furnace body a pressurized condition.

CONSTITUTION: Atomized heavy oil is supplied into a furnace body 12 from a burner 14 while combustion assisting air is supplied from a combustion assisting gas supplying means into the furnace body 12. In this case, combustion assisting air is constituted so as to be hardly discharged out of an exhaust port 11 (the exhausting port 11 is formed so as to have a size making the inside of the furnace body 12 a pressurized condition) whereby the inside of the furnace body is brought into pressurized condition. When the atomized heavy oil is ignited, the combustion of the heavy oil is spread into the whole body in the furnace body 12 and the inside of the furnace body 12 becomes a high temperature, gas in the furnace body 12 is expanded and a pressure in the furnace body 12 becomes more high to completely burn the atomized heavy oil. As a result, the ash of incineration 1, supplied to the inside of furnace 12 is molten perfectly by a temperature in the furnace body 12. According to this method, the flame, generated by fuel, will never be extinguished and supplied ash of incineration 1 can be melted completely.