The water treatment market using RO membranes has been expanding. Biofouling has become one of the major issues in operating RO systems. Slime control agents are one of effective measure to control biofouling, in the other hand, optimization of their dosage is a big challenge. We have developed the Slime control agent Dosing Control System for RO, which calculates the optimum dosage of chemical according to the biofouling potential of raw water that fluctuates daily. As a result of the demonstration test, we have achieved optimization of membrane cleaning frequency and chemical costs for three projects.