APPARATUS AND METHOD FOR PRODUCING PURIFIED WATER

PROBLEM TO BE SOLVED: To provide an apparatus for producing purified water capable of efficiently producing a purified water having a low boron concentration.SOLUTION: There is provided an ultrapure water producing apparatus which comprises: an activated carbon device 1; a heater 2; a membrane type filtration device 3; a raw water tank 4; a pretreatment device 5; an electrically deionizing device 6; and a subtank 7 for primary purified water. Further, the pretreatment device 5 comprises: a first reverse osmosis membrane (RO) device 8; a second reverse osmosis membrane (RO) device 9; and a decarbonation membrane device 10. The pretreatment device 5 is designed so that treated water W1 having a chloride ion concentration of 100 ppb or less may be introduced into a desalination chamber in the electrically deionizing device 6 according to water quality of raw water W0.