To precipitate and remove 3-valent arsenic by incorporating a dithiocarbamic acid derivative or the salt of cyclic imine to improve the stability against acid.

A dithiocarbamic acid derivative of cyclic imine such as pyrrolidine dithiocarbamic acid, etc., or the sodium salt is incorporated in an arsenic immobilizing agent. For example, about 85 pts.wt. piperidine, about 80 pts.wt. sodium hydroxide and about 733 pts.wt. distilled water are charged in a four neck flask provided with a stirrer, a thermometer, a dropping funnel and a gas introducing pipe and heated to about 35°C. Carbon disulfide is dropped by about 152 pts.wt. while keeping 30-40°C. After the reaction is completed, carbon disulfide is removed by introducing gaseous nitrogen to obtain the arsenic immobilizing agent containing about 20 wt.% sodium piperidine dithiocarbamate. Arsenic is precipitated and removed stably even at pH 3 without generating gaseous hydrogen sulfide by treating the arsenic-containing waste water using the method.