PURPOSE: To effectively dry sludge by using metallic or ceramic balls as heat medium, heating the balls by incinerator waste gas, drying sludge by use of the heated balls and preheating the heat medium balls by incineration waste gas of the dried sludge.

CONSTITUTION: A preheating tower 1 is filled with heat medium balls (b), which are heated by incineration waste gas. In a rotating drum type dryer 2, heated heat medium balls (b) coming from the preheating tower 1 are mixed with sludge (a) coming from a sludge tank 3 in order to dry the sludge (a). In a separator 4, the mixture coming from the dryer 2 is separated into the heat medium balls (b) and the dried sludge (a). A conveyer means 5 returns the separated heat medium balls (b) to the preheating tower 1. An incinerator 6 incinerates the separated dried sludge (a) and odor constituents and send the generated incineration waste gas to the preheating tower 1.