In up-flow anammox reactors, one of the contributing factors of biomass wash-out is the adherence of nitrogen gas produced by anammox reaction to biomass. In this study, we operated an up-flow anammox reactor equipped with a degassing membrane to minimize the biomass wash-out from the reactor by separating the produced gas from the biomass. In addition, both the effect of degassing on the anammox reactor performance and the durability of the membrane submerged in the anammox reactor were investigated. The results show that the use of the degassing membrane in the anammox reactor could 1) Improve the biomass retention ability (by separating the produced gas from the biomass), and 2) Increase the component ratio of anammox bacteria in the reactor. In addition, degassing could reduce the N2O emission produced in the reactor (for the gas selectivity of the degassing membrane). No membrane fouling was observed even after two months of operation without washing, indicating an advantage to the use of the degassing membrane.