A method for desalination for enabling hydrogen production, the method comprising: a desalination process using a desalination process unit in which influent is water-treated through a reverse osmosis process to produce produced water and concentrated water; and a chlor-alkali process using a chlor-alkali process unit having an interior space partitioned into an anode section to which the concentrated water is supplied and a cathode section (155, 156) to which the produced water is supplied, wherein the chlor-alkali process unit uses solar electrical energy to generate chlorine gas from salt of the concentrated water through a chemical reaction at the anode section with the concentrated water as an electrolyte, and generate hydrogen gas and sodium hydroxide through a chemical reaction at the cathode section (155, 156) with the produced water as an electrolyte, further comprising: a concentration process using a concentration process unit provided between the desalination process using a desalination process unit and the chlor-alkali process unit, and water-treating the concentrated water through a distillation process to produce a second concentrated water concentrated to a higher salinity than the concentrated water, and a second produced water that results from removal of the salt from the concentrated water, further comprising: a precipitation process unit provided between the concentration process unit and the chlor-alkali process unit, injecting the sodium hydroxide into the second concentrated water supplied from the concentration process unit, and water-treating the second concentrated water through a precipitation reaction to produce a third concentrated water with divalent cations removed, further comprising: a concentrated water pre-treatment unit provided between the precipitation process unit and the chlor-alkali process unit, injecting hydrochloric acid to the third concentrated water supplied from the precipitation process unit, and water-treating the third concentrated water to produce a fourth concentrated water having a lower pH than the third concentrated water, wherein the chlor-alkali process unit receives the fourth concentrated water from the concentrated water pre-treatment unit to the anode section, generates the chlorine gas in the fourth concentrated water through the chemical reaction at the anode section with the fourth concentrated water as an electrolyte, and discharges diluted concentrated water that results from removal of the salt from the fourth concentrated water to the concentrated water pre-treatment unit. a desalination process using a desalination process unit in which influent is water-treated through a reverse osmosis process to produce produced water and concentrated water; and a chlor-alkali process using a chlor-alkali process unit having an interior space partitioned into an anode section to which the concentrated water is supplied and a cathode section (155, 156) to which the produced water is supplied, wherein the chlor-alkali process unit uses solar electrical energy to generate chlorine gas from salt of the concentrated water through a chemical reaction at the anode section with the concentrated water as an electrolyte, and generate hydrogen gas and sodium hydroxide through a chemical reaction at the cathode section (155, 156) with the produced water as an electrolyte, further comprising: a concentration process using a concentration process unit provided between the desalination process using a desalination process unit and the chlor-alkali process unit, and water-treating the concentrated water through a distillation process to produce a second concentrated water concentrated to a higher salinity than the concentrated water, and a second produced water that results from removal of the salt from the concentrated water, further comprising: a precipitation process unit provided between the concentration process unit and the chlor-alkali process unit, injecting the sodium hydroxide into the second concentrated water supplied from the concentration process unit, and water-treating the second concentrated water through a precipitation reaction to produce a third concentrated water with divalent cations removed, further comprising: a concentrated water pre-treatment unit provided between the precipitation process unit and the chlor-alkali process unit, injecting hydrochloric acid to the third concentrated water supplied from the precipitation process unit, and water-treating the third concentrated water to produce a fourth concentrated water having a lower pH than the third concentrated water, wherein the chlor-alkali process unit receives the fourth concentrated water from the concentrated water pre-treatment unit to the anode section, generates the chlorine gas in the fourth concentrated water through the chemical reaction at the anode section with the fourth concentrated water as an electrolyte, and discharges diluted concentrated water that results from removal of the salt from the fourth concentrated water to the concentrated water pre-treatment unit.