To provide a method of generating, with good reproducibility and ease and without complicated operations, scorodite which satisfies the elution standard (in accordance with Notification of No. 13 of Japanese Environment Agency) and which has good filterbility and stability for processing arsenic contained in a diarsenic trioxide form. A method of processing diarsenic trioxide, including: a leaching step of adding water to diarsenic trioxide to produce slurry, heating the slurry, and leaching arsenic while adding an oxidant to obtain leachate; a deoxidization step of removing the oxidant so as to obtain an adjusted solution; and a crystallizing step of converting arsenic in the adjusted solution to scorodite crystal.