To provide a method of removing boron in a reverse osmosis seawater desalination device in which method a filter membrane separation device compact and easy to maintain is used as a pretreatment of a reverse osmosis membrane device to highly and effectively remove boron and clean water for finish is not needed in a final process.

Raw seawater to which a pH modifier and a coagulant for forming a boron containing insoluble deposit are added is fed into a circulating water tank. A supernatant in the circulating water tank is filtered by a microfilter membrane or an ultrafilter membrane and unfiltered component concentrated water of the microfilter membrane or the ultrafilter membrane is circulated to the circulating water tank and filtered water is permeated through a reverse osmosis membrane, allowing fresh water which seawater gets rid of boron to form to be produced.