To restrain generation of harmful hexavalent chromium and dioxin by controlling the combustion condition of fuel and the scavenging conditions of flying ash without adding a processing chemical to the ash discharged from a combustion device and the flying ash in the combustion device using organic material containing chromium as fuel.

The organic material fuel containing chromium is reduced and burnt in the combustion device body, dust in unburnt gas generated by reduction combustion is scavenged by a high-temperature filter device, the scavenged high-temperature dust is quick-cooled and discharged to the outside, and further the unburnt gas from which the dust has been removed is completely burnt, thereby preventing inversion of trivalent chromium in the process of combustion to the hexavalent chromium and re-synthesis of dioxin after thermal decomposition.