A desalination apparatus includes a pretreatment device 13 having a pretreatment membrane 13a that filters suspended matters in raw water 11 with added chlorine-containing water 12, a reverse osmosis membrane device 17 having a reverse osmosis membrane 16 that removes a salt content from filtrate water 14 supplied from the pretreatment device 13 to produce permeated water 15, and a reducing-agent injection device 30 that neutralizes the added chlorine on an upstream side of the reverse osmosis membrane device 17. The reducing-agent injection device 30 extracts a part 14a of the filtrate water 14, adds metering SBS solution 18 to the part, obtains a maximum change point (equivalence point) of the oxidation-reduction potential of a reducing agent and chlorine, obtains reducing-agent additive concentration (Csm) corresponding to the equivalence point, and supplies the SBS solution 18 corresponding to the concentration to the filtrate water 14 to thereby neutralize the chlorine.