In a submerged membrane activated sludge process that uses hollow fiber microfiltration for separation of activated sludge and the effluent, the suction pressure and the filtration resistance increase due to clogging of micro pores of the membrane and adhesion of sludge solids to the membrane surface, and the efficiency of the process decreases. In this study, wastewater treatment experiments were conducted with a wide range of initial sludge solids concentration (MLSS) under low BOD volumetric loading condition and constant membrane permeation flux condition to determine the effect of MLSS on various filtration resistances. Suction pressure was found to increase rapidly when MLSS exceeds 12000 mg/L. In this condition, adhesion of sludge solids to the membrane surface increased. The filtration resistance of cake is proportional to the amount of adhering sludge solids, and the solid liquid separation process is based on Ruth's filtration equation. Therefore, for the long and stable operation of the submerged membrane activated sludge process, it is recommended that the process should be operated at MLSS of less than 12000 mg/L.