A water treatment device, comprising: a magnesium ion adding section that is configured for adding a magnesium ion additive to water to be treated; a pH adjusting agents adding section that is arranged downstream of the magnesium ion adding section and that is configured for adding pH adjusting agents to the water to be treated containing soluble silica, to thereby adjust the pH of the water to be treated to a pH of 5.5 or higher; a soluble silica deposition section that is arranged downstream of the pH adjusting agents adding section and that is configured for depositing soluble silica dissolved in the water to be treated; an aluminate ion adding section that is configured for adding sodium aluminate as an aluminate ion additive represented by general formula [Al 4] - to the water to be treated in the soluble silica deposition section, to thereby adjust the aluminate ion concentration in the water to be treated in such a manner that the concentration ratio of aluminum ions to the soluble silica (mol Al/mol SiO 2) is 0.5 or higher; a seed material adding section that is configured for adding soluble silica that is deposited in advance from the water to be treated, as a seed material, to the water to be treated in the soluble silica deposition section; a solid-liquid separating section that is arranged downstream of the soluble silica deposition section and that is configured for separating the deposited soluble silica from the water to be treated, to thereby obtain treated water formed by removing the soluble silica from the water to be treated; and a treated water purifying section that is arranged downstream of the solid-liquid separating section and that includes at least one type selected from the group consisting of: an evaporating section that is configured for obtaining purified treated water by evaporating the treated water; a crystallizing section that is configured for obtaining purified treated water by crystallizing the treated water; and an ion exchange resin section that is configured for obtaining purified treated water by ion-exchanging the treated water. a magnesium ion adding section that is configured for adding a magnesium ion additive to water to be treated; a pH adjusting agents adding section that is arranged downstream of the magnesium ion adding section and that is configured for adding pH adjusting agents to the water to be treated containing soluble silica, to thereby adjust the pH of the water to be treated to a pH of 5.5 or higher; a soluble silica deposition section that is arranged downstream of the pH adjusting agents adding section and that is configured for depositing soluble silica dissolved in the water to be treated; an aluminate ion adding section that is configured for adding sodium aluminate as an aluminate ion additive represented by general formula [Al 4] - to the water to be treated in the soluble silica deposition section, to thereby adjust the aluminate ion concentration in the water to be treated in such a manner that the concentration ratio of aluminum ions to the soluble silica (mol Al/mol SiO 2) is 0.5 or higher; a seed material adding section that is configured for adding soluble silica that is deposited in advance from the water to be treated, as a seed material, to the water to be treated in the soluble silica deposition section; a solid-liquid separating section that is arranged downstream of the soluble silica deposition section and that is configured for separating the deposited soluble silica from the water to be treated, to thereby obtain treated water formed by removing the soluble silica from the water to be treated; and a treated water purifying section that is arranged downstream of the solid-liquid separating section and that includes at least one type selected from the group consisting of: an evaporating section that is configured for obtaining purified treated water by evaporating the treated water; a crystallizing section that is configured for obtaining purified treated water by crystallizing the treated water; and an ion exchange resin section that is configured for obtaining purified treated water by ion-exchanging the treated water.