PURPOSE: To reduce sexivalent chromium in combustion ashes into trivalent chromium and take out combustion ashes in lumps, by operating flow-combustion on chromium-containing and dehydrated sludge and bringing the combustion ashes into contact with reducing gas.

CONSTITUTION: An air flow passes through inlet pipe 11 and inlet header 11A and is supplied to head unit 7 from dispersion nozzle 12 inside flow combustion furnace proper 5, and thereby flow medium 21 is caused to flow. Sludge containing chromium, auxiliary fuel and secondary air are supplied respectively from inlets 14, 13 and 15. Sludge, together with auxiliary fuel, is made to flow and gets oxygen and is burned. Combustible materials in head unit 7 enter free board unit 6 subsequently and are completely burned. Then, they are discharged from outlet 16. On the other hand, combustion ashes are brought into contact with reducing gas from reducing gas inlet 10 in reducing unit 8 on the lower part of the flow layer. By this, sexivalent chromium in combustion ashes is reduced to trivalent chromium and is discharged in lumps from discharge unit 9.