PURPOSE: To perform a reignition of dusts and improve a reduction of waste materials by a method wherein almost of all the oil coke dust are supplied to a combustion chamber together with stabilizing oil fed to the combustion chamber.

CONSTITUTION: Combustion gas is circulated up and down in a main body 9 of a boiler and when a descending flow is converted into an ascending flow, dust 11 is removed under a centrifugal force and then dust 11a is accumulated in a hopper 12. Further, in a dust collector 2, dust is removed in a multiple step manner, the dust to be wasted is removed and then fed with air into a mixing machine 19 under operation of a reignition fan 15. The dust collected in this way is directly returned back into the combustion chamber 3 through a stabilizing burner 17 from an oil supplying pipe 18. With this arrangement, a large amount of dust generated under ignition of oil coke is performed and a saving of resource and saving of energy are realized and at the same time it is possible to make a substantial reduction of an amount of waste material.