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 non-ferrous smelting intermediate, particularly, for processing a diarsenic trioxide form. A method of processing diarsenic trioxide, including: a leaching step of adding water and alkali to a non-ferrous smelting intermediate that contains diarsenic trioxide to produce slurry, heating the slurry, and leaching arsenic; a solution adjusting step of adding an oxidizing agent to the leachate to oxidize trivalent arsenic to pentavalent arsenic so as to obtain an adjusted solution; and a crystallizing step of converting arsenic in the adjusted solution to scorodite crystal.