A water treatment process, comprising: a first scale inhibitor supplying step of supplying a calcium scale inhibitor which is a scale inhibitor for inhibiting the deposition of a scale containing calcium and a silica scale inhibitor which is a scale inhibitor for inhibiting the deposition of silica to water to be treated containing Ca ions, SO 4 ions, carbonate ions and silica; a first demineralizing step of separating the water to be treated into first concentrated water in which the Ca ions, the SO 4 ions, the carbonate ions and the silica are concentrated and treated water after the first scale inhibitor supplying step; a first crystallizing step of supplying seed crystals of gypsum to the first concentrated water so that gypsum is crystallized from the first concentrated water; a second scale inhibitor supplying step of supplying the calcium scale inhibitor to the water to be treated after the first crystallizing step; a second demineralizing step of separating the water to be treated into second concentrated water in which the Ca ions, the SO 4 ions, the carbonate ions and the silica are concentrated and treated water after the second scale inhibitor supplying step; and a second crystallizing step of supplying seed crystals of gypsum to the second concentrated water so that gypsum is crystallized from the second concentrated water, characterized in that the water treatment process further comprises: a first pH adjusting step of adjusting the first concentrated water to pH 6.0 or less at which a scale inhibition function of the calcium scale inhibitor is reduced between the first demineralizing step and the first crystallizing step, thereby promoting the deposition of the gypsum in the first crystallizing step; and a second pH adjusting step of adjusting the water to be treated to pH 10 or more at which the silica is soluble in the water to be treated by adding alkali thereto after the first crystallizing step and before the second demineralizing step. a first scale inhibitor supplying step of supplying a calcium scale inhibitor which is a scale inhibitor for inhibiting the deposition of a scale containing calcium and a silica scale inhibitor which is a scale inhibitor for inhibiting the deposition of silica to water to be treated containing Ca ions, SO 4 ions, carbonate ions and silica; a first demineralizing step of separating the water to be treated into first concentrated water in which the Ca ions, the SO 4 ions, the carbonate ions and the silica are concentrated and treated water after the first scale inhibitor supplying step; a first crystallizing step of supplying seed crystals of gypsum to the first concentrated water so that gypsum is crystallized from the first concentrated water; a second scale inhibitor supplying step of supplying the calcium scale inhibitor to the water to be treated after the first crystallizing step; a second demineralizing step of separating the water to be treated into second concentrated water in which the Ca ions, the SO 4 ions, the carbonate ions and the silica are concentrated and treated water after the second scale inhibitor supplying step; and a second crystallizing step of supplying seed crystals of gypsum to the second concentrated water so that gypsum is crystallized from the second concentrated water, characterized in that the water treatment process further comprises: a first pH adjusting step of adjusting the first concentrated water to pH 6.0 or less at which a scale inhibition function of the calcium scale inhibitor is reduced between the first demineralizing step and the first crystallizing step, thereby promoting the deposition of the gypsum in the first crystallizing step; and a second pH adjusting step of adjusting the water to be treated to pH 10 or more at which the silica is soluble in the water to be treated by adding alkali thereto after the first crystallizing step and before the second demineralizing step. a first pH adjusting step of adjusting the first concentrated water to pH 6.0 or less at which a scale inhibition function of the calcium scale inhibitor is reduced between the first demineralizing step and the first crystallizing step, thereby promoting the deposition of the gypsum in the first crystallizing step; and a second pH adjusting step of adjusting the water to be treated to pH 10 or more at which the silica is soluble in the water to be treated by adding alkali thereto after the first crystallizing step and before the second demineralizing step.