PURPOSE: To reduce the generation of the unburned combustibles due to incom plete combustion by adjusting the water content of sludge so that the measured value of the electrolytic current is controlled within specified limits and then supplying the sludge to an incinerator.

CONSTITUTION: The sludge in a quantitative feed tank 5 is discharged on a belt conveyor 3 in fixed amts., conveyed by a feed screw 2, supplied into a multi-stage incinerator 1 from its top by the screw 2 and incinerated. Meanwhile, a rotating electrode 6 is inserted into the sludge on the conveyor 3 to continuously measure the constant-voltage electrolytic current, and the measured value is inputted to a computing element 7 through a moisture meter 4. The amt. of water to be supplied is calculated to fix the water content of sludge, the opening degree of a water feed control valve 8 is adjusted by the signal, and water is sprinkled into the tank 5 from the tip of a water feed pipe 9. Consequently, sludge having a fixed water content is supplied to the incinerator, and the sludge is stably burned.