PURPOSE: To effectively and easily separate oil and foreign matter in a sludge, by adding a chemical contg. a silicate and a polymer to an oil-contg. sludge and flocculating the sludge by causing aggregation and adsorption followed by pressing under heating.

CONSTITUTION: An oil-contg. sludge is introduced via a dust collector 2 and a tank 4 into a chemical-injected agitator 6, in which firm porous floc is formed by injecting a chemical 6a contg. a silicate and a polymer under agitation to cause aggregation and adsorption. The flocculated sludge is pressed with a screw press 7 under heating and separated into a solid cake and a liquid phase. The liquid phase is introduced into an oil/water separator 8 and separated into oil, water and floc. The floc is returned again to the agitator 6, and the oil and water are introduced respectively into a waste oil disposer and waste water disposer. The solid cake is carried by a conveyer 9 and modified into a fuel.