A desalination apparatus comprising: a line for flowing raw water with added chlorine-containing water; an extraction line for extracting a part of the raw water from an extraction position of the line as an extracted water; a reducing-agent tank for adding a reducing agent solution to the line downstream of the extraction position so as to neutralize chlorine of the raw water; a control valve provided between the line and the reducing-agent tank, for controlling an additive amount of the reducing agent solution in response to a flow regulation signal from a flow controller; a reverse osmosis membrane device including a reverse osmosis membrane configured to remove a salt content from raw water to thereby produce permeated water, the reverse osmosis membrane device being provided downstream of a supply position of the reducing agent solution; a metering reducing-agent supply tank configured to supply a metering reducing agent solution to the extraction line; a sequencer configured to supply a metering reducing-agent supply signal having a pattern of change; a flow-variable metering pump provided between the extraction line and the metering reducing-agent supply tank, for changing a supply flow rate of the metering reducing agent solution in response to the metering reducing-agent supply signal; an oxidation-reduction potential meter configured to measure an oxidation-reduction potential of the extracted water of the extraction line downstream of a supply position of the metering reducing agent solution and to produce an oxidation-reduction potential signal; and an arithmetic device configured to: receive the metering reducing-agent supply signal and the oxidation-reduction potential signal, based on the metering reducing-agent supply signal and the oxidation-reduction potential signal, obtain an equivalence point which is a maximum change point of the oxidation-reduction potential of the metering reducing agent solution and chlorine of the extracted water; obtain a reducing-agent additive concentration (Csm) corresponding to the equivalence point; obtain the additive amount of the reducing agent solution corresponding to the reducing-agent additive concentration (Csm); and output the flow regulation signal via the flow controller to the control valve. a line for flowing raw water with added chlorine-containing water; an extraction line for extracting a part of the raw water from an extraction position of the line as an extracted water; a reducing-agent tank for adding a reducing agent solution to the line downstream of the extraction position so as to neutralize chlorine of the raw water; a control valve provided between the line and the reducing-agent tank, for controlling an additive amount of the reducing agent solution in response to a flow regulation signal from a flow controller; a reverse osmosis membrane device including a reverse osmosis membrane configured to remove a salt content from raw water to thereby produce permeated water, the reverse osmosis membrane device being provided downstream of a supply position of the reducing agent solution; a metering reducing-agent supply tank configured to supply a metering reducing agent solution to the extraction line; a sequencer configured to supply a metering reducing-agent supply signal having a pattern of change; a flow-variable metering pump provided between the extraction line and the metering reducing-agent supply tank, for changing a supply flow rate of the metering reducing agent solution in response to the metering reducing-agent supply signal; an oxidation-reduction potential meter configured to measure an oxidation-reduction potential of the extracted water of the extraction line downstream of a supply position of the metering reducing agent solution and to produce an oxidation-reduction potential signal; and an arithmetic device configured to: receive the metering reducing-agent supply signal and the oxidation-reduction potential signal, based on the metering reducing-agent supply signal and the oxidation-reduction potential signal, obtain an equivalence point which is a maximum change point of the oxidation-reduction potential of the metering reducing agent solution and chlorine of the extracted water; obtain a reducing-agent additive concentration (Csm) corresponding to the equivalence point; obtain the additive amount of the reducing agent solution corresponding to the reducing-agent additive concentration (Csm); and output the flow regulation signal via the flow controller to the control valve. receive the metering reducing-agent supply signal and the oxidation-reduction potential signal, based on the metering reducing-agent supply signal and the oxidation-reduction potential signal, obtain an equivalence point which is a maximum change point of the oxidation-reduction potential of the metering reducing agent solution and chlorine of the extracted water; obtain a reducing-agent additive concentration (Csm) corresponding to the equivalence point; obtain the additive amount of the reducing agent solution corresponding to the reducing-agent additive concentration (Csm); and output the flow regulation signal via the flow controller to the control valve.