PURPOSE: To simply and nicely clean a grease filter without contaminating the hand by placing the grease filter under temp.and hunidity environment suitable for fermentation bacteria and decomposing the oils and fats bonded to the grease filter by fermentation bacteria.

CONSTITUTION: A heat generating agent 7 is separately received in a plurality of the divided sections 8 of a cleaning bag 6 made possible to receive a grease filter 1 by a predetermined amount and, after fermentation bacteria received in a fermentation bacteria receiving bag 12 are transferred to the cleaning bag 6, the seal material of the cleaning bag 6 is peeled off to lightly shake the cleaning bag 6 so as to be capable of allowing the heat generating agent to combine with oxygen. After the heat generating agent 7 beings to generate heat, the grease filter 1 is put in the cleaning bag 6 and a lid 10 is folded to be fixed by a self-adhesive tape 11. When a predetemined time is elapsed, the oils and fats bonded to the grease filter 1 are wholly decomposed to be converted to a high-molecular compound having a higher MW. The grease filter 1 is taken out of the cleaning bag 6 to be washed and the cleaning bag 6 is again closed by the lid 10 after use to be discarded as it is.