According to one embodiment, water to be treated is passed through each of a first reactor vessel and second reactor vessel, phosphorus contained in the water is removed by using an adsorbent, thereafter the liquid held in each of the reactor vessels is discharged from each of a first treated water discharge path and second treated water discharge path, and thus the influence of soluble impurities derived from the water is eliminated. Further, when suspended solids impurities contained in the water are accumulated in the phosphorus adsorbent, the accumulated impurities are eliminated by a reverse cleaning operation using a liquid to be thereafter introduced from a reactor vessel liquid introduction path into the reactor vessel, and hence the influence of the suspended solids in the water on a desorption agent liquid to be thereafter introduced into a crystallization vessel, and containing phosphorus from the phosphorus adsorbent is eliminated.