PURPOSE: To effectively recover energy to utilize the same, by incinerating a part of the dehydrated cake of org. sludge after drying, and drying the remaining dehydrated cake by heat energy generated by incineration.

CONSTITUTION: Sewage sludge 1 is dehydrated by a dehydrator 2 to obtain a dehydrated cake 1 and a part 4 thereof is sent to a dryer 7 and dried by the holding heat of air 6 emitted from an aeration blower 5 so as to adjust the moisture content thereof to about 10W20% to form sludge fuel 8. In this case, emitted air 6' of which the temp. is fallen through the use in the dryer 7 is supplied to the aeration tank 10 of an activated sludge treating process of sewage 9. The sludge fuel 8 is supplied to an incinerator 11 to be incinerated and the generated heat energy is recovered to be supplied to a dryer 13 where the remainder 4' of the dehydrated cake 3 is dried to form sludge fuel 14. This sludge fuel 14 is incinerated by an incinerator 15 and the recovered energy 16 is effectively utilized in the other facilities.