A method for operating a reverse osmosis membrane filtration plant having a raw water intake unit, a pre-treatment unit, and a reverse osmosis membrane filtration unit having a reverse osmosis membrane module in this order, said method comprising: disposing a reverse osmosis membrane used as a biofilm formation base material in a flow container, wherein the reverse osmosis membrane is a rectangular sheet and is rolled in such a manner that a separation function layer surface serves as the inner side and the rolled reverse osmosis membrane is pushed into the flow container along an inner wall of the flow container wherein the inner side is the part on which the evaluation water inside the flow container flows, so that reverse osmosis membrane module supply water sampled from piping branching at upstream of the first reverse osmosis membrane module in the reverse osmosis membrane filtration unit and/or reverse osmosis membrane module non-permeated water sampled from piping branching at downstream of the reverse osmosis membrane modules through which the reverse osmosis membrane module non-permeated water passes, are/is flowed on a surface of a separation function layer of the reverse osmosis membrane sheet used as a biofilm formation base material housed in the flow container, and so that the reverse osmosis membrane module supply water and/or the reverse osmosis membrane module non-permeated water are/is not filtered by the reverse osmosis membrane sheet used as the biofilm formation base material housed in the flow container but under conditions that the reverse osmosis membrane module supply water and/or the reverse osmosis membrane module non-permeated water are/is flowed at a linear speed equal to a non-permeated water linear speed in the reverse osmosis membrane module of the reverse osmosis membrane filtration unit and equal to an average linear speed on the surface of the reverse osmosis membrane module on which the non-permeated water is flowed; evaluating a biofilm amount on the reverse osmosis membrane sheet used as the biofilm formation base material housed in the flow container at a frequency of from once a day to once in six months; and controlling the method for operating a reverse osmosis membrane filtration plant based on results of the evaluation, wherein the reverse osmosis membrane used as the biofilm formation base material is made of the same material as a reverse osmosis membrane which is used in the reverse osmosis membrane filtration plant. disposing a reverse osmosis membrane used as a biofilm formation base material in a flow container, wherein the reverse osmosis membrane is a rectangular sheet and is rolled in such a manner that a separation function layer surface serves as the inner side and the rolled reverse osmosis membrane is pushed into the flow container along an inner wall of the flow container wherein the inner side is the part on which the evaluation water inside the flow container flows, so that reverse osmosis membrane module supply water sampled from piping branching at upstream of the first reverse osmosis membrane module in the reverse osmosis membrane filtration unit and/or reverse osmosis membrane module non-permeated water sampled from piping branching at downstream of the reverse osmosis membrane modules through which the reverse osmosis membrane module non-permeated water passes, are/is flowed on a surface of a separation function layer of the reverse osmosis membrane sheet used as a biofilm formation base material housed in the flow container, and so that the reverse osmosis membrane module supply water and/or the reverse osmosis membrane module non-permeated water are/is not filtered by the reverse osmosis membrane sheet used as the biofilm formation base material housed in the flow container but under conditions that the reverse osmosis membrane module supply water and/or the reverse osmosis membrane module non-permeated water are/is flowed at a linear speed equal to a non-permeated water linear speed in the reverse osmosis membrane module of the reverse osmosis membrane filtration unit and equal to an average linear speed on the surface of the reverse osmosis membrane module on which the non-permeated water is flowed; evaluating a biofilm amount on the reverse osmosis membrane sheet used as the biofilm formation base material housed in the flow container at a frequency of from once a day to once in six months; and controlling the method for operating a reverse osmosis membrane filtration plant based on results of the evaluation,