To develop a practical method of treating boron-containing wastewater, we custom designed two laboratory-scale continuous-flow-type hydrothermal mineralization apparatuses. We carried out a hydrothermal treatment using a continuous-flow apparatus, using Ca(OH)₂, CaCl₂, and both Ca(OH)₂ and CaCl₂ as mineralizers, under various pH conditions. Our results show that using CaCl₂ under optimal pH conditions (pH = approximately 12) allowed for the stable removal of boron from wastewater. One of the precipitates obtained in this process was parasibirskite, which is a natural boron mineral. Finally, because the residual concentration of boron could not be reduced below 10 mg/dm³, we propose a realistic boron removal wastewater treatment system incorporating the hydrothermal mineralization method.