A cement kiln from which its brick lining has been removed, or a rotary dryer for cement production is used as a fermentation treatment apparatus 202 for converting waste matter into compost. The waste matter to be subjected to the fermentation treatment includes city waste contained in garbage bags, and its fermentation treatment is carried out by introducing the waste matter directly into the aforesaid cement kiln or the aforesaid rotary dryer for cement production without tearing the aforesaid garbage bags.