PURPOSE: To efficiently reduce the concn. of the malodor contained in exhaust air to a lower level at a low running cost.

CONSTITUTION: Bacteria decomposing raw dust are added to a moisture absorbing member 1 and raw dust is added to this moisture absorbing member 1 to be stirred along with the moisture adsorbing member 1 to be decomposed by the action of bacteria. The malodorous gas generated by the decomposition of the raw dust is passed through a porous material 10 held to a wet state and a malodorous component is adsorbed by the porous material 10. Further, bacteria decomposing the malodorous component adsorbed on the porous material 10 are allowed to live and the malodorous component adsorbed on the porous material 10 is decomposed by bacteria to prevent the lowering of the adsorbing capacity of the porous material 10.