To control concentration of lead contained in slag.

A waste treatment device is provided with: a thermal decomposition drum 1 for cracking waste 3; a combustion melting furnace 5 for burning cracked gas exhausted from the thermal decomposition drum 1 and melting at least noncombustible solid matter entrained in the cracked gas; a high temperature air heater 9 recovering fly ash contained in the exhaust gas exhausted from the combustion melting furnace 5; a waste heat boiler 11; an attemperating tower 15; and a bag filter 17; and a change-over damper 27 provided in a conveyance path conveying the fly ash recovered by the high temperature air heater 9, the waste heat boiler 11, the attemperating tower 15 and the bag filter 17 and extracting the fly ash in the conveyance path. By changing the change-over damper 27 to an exhaust side and adjusting a supply amount inputted in the combustion melting furnace when concentration of sulfur oxides or hydrogen chloride of the exhaust gas is a set value or more, the concentration of lead contained in the slag 7 is controlled.