There is a phenomenon observed in various microbial waste treatment processes, in which the microbial degradation rate of organic substrates drastically decreases when the input of substrates to the process is too heavy. It is called "overloading". The overloading phenomenon in garbage treatment systems often causes serious problems. But the mechanism of overloading has not yet been clarified. This study intends to investigate this phenomenon by analyzing the microbial community as well as various physical parameters in the garbage treatment process.