PURPOSE: To quickly and efficiently produce solid matter by heating sludge under strong agitation in an agitation tank having an impeller in the tank bottom and subjecting the sludge to continuous dehydrating, pelletizing and drying within the same tank.

CONSTITUTION: A binder essentially consisting of coal ash is added at 5W50wt% ratio of the entire part to the combustible sludge which is the activated sludge generated in a biological treatment of waste water or the digester sludge of the activated sludge or pulp waste or waste liquid of fermentation or waste liquid of food processing or the residue of animal and vegetable food processing. While the sludge is strongly agitated in the agitation tank which has the impeller in the tank bottom and has or does not have a baffle plate in the tank, the sludge is subjected to indirect or direct heating or simultaneous direct and indirect heating. The sludge is thus continuously dehydrated, pelletized and dried within the same tank. As a result, the solid matter made of the sludge is quickly and efficiently produced.