This filtration treatment system is provided with a membrane filtration device as a prefiltration treatment device for removing insoluble components from the water to be treated containing organic substance, a reverse osmosis membrane treatment device for subjecting the prefiltration-treated water to reverse osmosis membrane treatment, and a chemical agent supply pipe for supplying a chemical agent into the membrane filtration device, wherein the chemical agent contains a sulfamic acid compound, and a bromine-based oxidizing agent or the reaction product of a bromine compound with a chlorine-based oxidizing agent, or, contains the reaction product of a sulfamic acid compound with a bromine-based oxidizing agent or with the reaction product of a bromine compound with a chlorine-based oxidizing agent.