PROBLEM TO BE SOLVED: To provide a pretreatment method of a reverse osmosis membrane device in which water treatment effect due to oxidation chemicals is effectively obtained by preventing RO membrane deterioration caused by a remained oxidizer by the addition of a reducer and preventing the decomposition of the oxidation chemicals caused by the added reducer when performing RO membrane treatment by adding the oxidation chemicals to water containing the oxidizer.SOLUTION: In a pretreatment method of a reverse osmosis membrane device,: nitrite and/or a salt of nitrite is used as a reducer; nitrite and/or a salt of nitrite reduces a remained oxidizer such as chlorine and the like and does not perform an oxidation reduction reaction with oxidation chemicals such as chlorosulfamic acid and the like in practical use concentration if an excess remains; and effective component concentration is not reduced when the oxidation chemicals are reduced and removed.SELECTED DRAWING: Figure 1
