To provide a method of treating by which a highly crystalline scorodite compound and a filtrate to be returned to the arsenic-containing liquid before treatment to be repeatedly used are obtained from the arsenic-containing liquid.

In the precipitation and deposition of an iron arsenic compound by adding an oxidizing agent such as gaseous oxygen into the arsenic-containing liquid containing arsenic ion and bivalent iron ion and stirring, a Fe/As molar ratio of the liquid (a liquid before reaction) before the start-up of the precipitation reaction is controlled to 0.8-1.0, while the pH before reaction is controlled to 0-2.0 and a neutralization agent or the oxidizing agent is added to accelerate the precipitation reaction during the precipitation reaction progression after an acid is started to be formed with the advance of precipitation reaction. The total quantity of the neutralization agent or the oxidizing agent to be added is controlled so that the concentration of iron in the filtrate (liquid after reaction) obtained by solid-liquid separating slurry after the precipitation reaction is finished is 2.0 g/L and the pH after precipitation reaction finishing is 1.2.