For the oxidation of low concentrations of NH4+-N, conventional biofilm reactors such as a rotating biological contactor encounter difficulty due to mass transport limitation of NH4+-N. Therefore, the authors have developed a novel biofilm-membrane reactor, in which biomass is fixed on the surface of rotating membrane disks to enhance NH4+-N transport into the biofilm. Three long-term bench-scale experiments were carried out and sufficient nitrification efficiency was obtained even at low levels of NH4+-N. The experimental results were evaluated in comparison with model simulation.