Abstract: In a seawater treatment method in which after seawater is coagulated by a coagulant and is processed by a solid-liquid separation treatment, an RO treatment is performed, RO feed water having a good water quality is obtained by a small addition amount of the coagulant, and by the RO treatment of the RO feed water, a stable RO treatment can be performed over a long period of time. In a seawater treatment method which is a pretreatment method performed prior to a membrane separation treatment of seawater, after a cationic organic flocculant and/or an inorganic coagulant is added to seawater to conduct a reaction, a coagulation treatment is performed by addition of an alkaline solution of a high molecular weight compound which has a phenolic hydroxide and which is insolubilized under a high salt concentration, and a solid-liquid separation treatment is then performed. Subsequently, this treated water is desalinated by a membrane separation treatment.