The authors have proposed a novel membrane process for drinking water treatment in which nitrifiers are fixed on the membrane surface. With this proposed membrane process, strict solid-liquid separation by membrane filtration and oxidation of ammonia can be performed simultaneously. Fixed nitrifiers are supposed to cause membrane filtration resistance to some extent. The magnitude of the filtration resistance caused by the fixed nitrifiers for oxidizing low concentrations of ammonia was evaluated in this study. Experimental results showed that the fixation of the nitrifiers on the membrane surface had a little influence on the increase of the filtration resistance. Most of the resistance was caused by trace inorganic and organic substances contained in the feed water (tap water). The filtration resistance was divided into the cake resistance and the irreversible fouling resistance. The nitrifiers for the oxidation of low concentrations of ammonia seemed to increase the cake resistance a little and hardly cause the irreversible fouling.