Nanofiltration (NF) membrane technology has made rapid progress and then extended its application fields. It has obtained in particular the good results in the removal of organic and inorganic matters or microbes in the water and wastewater. To investigate the applicability and characteristics of NF membrane, the membrane bioreactor (MBR) using the cellulose acetate NF membrane was performed to treat synthetic wastewater. A hollow-fiber-type cellulose acetate membrane was chosen to get enough water productivity regardless of its biodegradability. As a result of the experiments, enough water productivity was obtained for 60 days without fatal fouling and membrane cleaning. This raised the applicability of cellulose acetate membrane to the MBR system. Electrolytes were not accumulated in the bioreactor, as its rejection was also low. This enabled the NF MBR to be operated under a low suction pressure and prevented from inhibition against microorganisms, whose activity might be deteriorated by high salt concentration. Nitrification and denitrification happened simultaneously and this might be achieved by the module configuration. The phosphorus was not removed with the loose NF membrane. According to AFM investigation, the cellulose acetate membrane after 71 days has larger voids because of the biodegradation.