Refractory dissolved organic matter (RDOM) is defined as the DOM which remains after 100 days biodegradation test of water samples. In order to know the transition of bacterial community, which is the degrader of DOM, we carried out the 100 days biodegradation experiments using lake water of Lake Biwa, Kasumigaura, Inba, Tega, and extracted DNA before and after the cultivation to conduct ampli-con sequencing analysis.