Abstract: Boric acid is recovered from a water phase separated after hydrolyzing the reaction mixture obtained by oxidizing cyclohexane in the presence of a catalyst of boric acid. The method comprises(a) a concentration-crystallization step of concentrating the water phase by heating it and crystallizing boric acid;(b) a concentrating step of separating crystals of boric acid from the slurry obtained by the step (a) and then controlling the water content of the mother liquor to 20 to 50 wt. %;(c) a cooling-crystallization step of cooling the concentrated solution obtained by the step (b) under a reduced pressure to crystallize boric acid; and(d) a boric acid recycling step of separating crystals of boric acid separated from the slurry obtained by the step (c) and then, recycling the boric acid to the step (a).