PURPOSE: To prevent the production of sexivalent chrome by forming an oxydizing layer, reducing layer and combusting layer in this order from the lower part of the incinerator by means of coal supplied into the incinerator.

CONSTITUTION: A large amount of coal 2 is charged inside of the incinerator 3 and fired by an ignition means. The coal 2 comes into combustion and when the combustion becomes stable, the oxydizing layer 9, reducing layer 10 and combusting layer 11 are formed in this order from the lower part of the incinerator 3. In the reducing layer 10, a reductive gas having reductive intermediate products such as carbon monoxide, hydrogen, hydrocarbon and so on is produced to prevent the oxidation of trivalent chrome and also reduce the sexivalent chrome to trivalent chrome. Thus, the production of the sexivalent chrome can be restrained effectively.