To efficiently feed necessary and sufficient air when subjecting granular substances such as ceramic granules in which microorganisms are mixed to rotation in a rotary drum and feeding garbage into them to bring it in mixing contact with the microorganisms well to make it extinct by fermentation.

In the peripheral wall 5a of a rotary drum 5 into which material to be agitated S is fed, slit holes 8 are made successively having cut length in the axial direction of the rotary drum. The slit holes 8 have upstream edges thereof in the rotation direction of the drum projected in louvers toward into the drum to form blades 8a. An air flow A sent from a fan 11 disposed near the rotary drum 5 is efficiently taken in the drum 5. And the slit holes 8 are prevented from being clogged by the material to be agitated S.