This study examined the feasibility of applying a membrane bioreactor to small wastewater treatment using a pilot-scale plant. The operation was continued for 336 days. From the start to 251st day, the operation was continued without membrane washing. During the latter term of the operation, however, suction pressure increased rapidly corresponding to the increase of sludge viscosity. Because sludge viscosity increase occurred right after the suspension of sludge wasting, excess sludge must be wasted at regular intervals in order to keep low sludge viscosity. More than 90 % of organic matter, suspended solids, and coliform bacteria were successfully removed. As far as nitrogen removal was concerned, denitrification was the rate determining step in this system. To enhance denitrification, the following three points should be considered: i) to concentrate inflow of raw sewage at the beginning of anoxic period, ii) to shorten aeration time, and iii) to maintain high MLSS concentration.