The water treatment system includes: an aluminate ion addition part for adding an aluminate ion additive to water to be treated containing at least salt content and silica; a first precipitation part provided on the downstream side of the aluminate ion addition part; a first pH adjustment part provided on the downstream side of the first precipitation part; a first solid/liquid separation part for separating the solid content in the water to be treated; a second pH adjustment part for adjusting the pH of the water to be treated from the first solid/liquid separation part to a second pH level; and a first separation membrane (desalination treatment) apparatus provided on the downstream side of the second pH adjustment part so as to separate the water to be treated into first permeated water and first non-permeated water by removing the salt content in the water to be treated.