The spiral wound type RO (reverse osmosis) membrane module (BW30-2540) for brackish water showed the high values of salt rejection rate and recovery rate in the primary performance tests. Therefore, it was selected for applying RO membrane process to groundwater treatment to use the industrial water. In the field test of the groundwater the value of permeate rate was almost constant, 29.93L/㎡·hr, and very stable water quality, TDS (total dissolved solids)=10∼20㎎/L. After the operation of the RO membrane process during 750hrs, the membrane was disassembled from the module and investigated to find the membrane fouling and its sources. The components of the fouling layer in the membrane surface were Si, Mn, Fe, Ca and K, which were supposed to be the source of the membrane fouling. And, the insoluble organic materials, i.e. algae, were accumulated on the membrane surface, and the inorganic salt component involved in the fouling layer was 1.2282%.