Abstract: Sludge which is supplied from an untreated sludge tank 3 and diluting water from a diluting water supply facility 14 is charged into a continuous kneading mixer 12 for diluting the sludge to make the specific weight of the sludge 1.15 to 1.35. The diluted sludge is supplied to a dehydrating machine 25 in accordance with the capacity of the used dehydrating machine 25. The diluted sludge is firstly added with an inorganic flocculating agent G and is mixed in a first blender 24A and is then added with a high molecular flocculating agent G2 and is mixed in a second blender 24B. Thereafter, the sludge is supplied to a belt press dehydrating machine 25 in which it is subjected to a dehydrating treatment.