A method that in the treatment of arsenic existing in the form of diarsenic trioxide, satisfies the elution standards (according to Notification No. 13 from the Environment Agency), ensures excellence in filtration and can form stable scorodite readily with high reproducibility without any complex operation. Diarsenic trioxide is treated by a treatment method comprising the leaching step of adding water to diarsenic trioxide into a slurry, warming the slurry and leaching out arsenic while adding an oxidizing agent to thereby obtain an leach liquor; the deoxidizing step of removing any oxidizing agent remaining in the leach liquor to thereby obtain a preparation liquid; and the crystallization step of converting the arsenic contained in the preparation liquid to a scorodite crystal.
