To make an operation stable and high efficiency and further improve quality of slag-metal upon melting/reducing processing for metal sludge.

Before metal sludge is blown into a melting/reducing furnace, part of the metal sludge after it is dried is collected, and the amount of remaining water in the sludge is measured, and further the amount of supply heat to the melting/reducing furnace is adjusted in response to the amount of the measured remaining water.