PURPOSE: To detach and recover magnetic particles by a small amount of treating water by a mechanism wherein a gas is introduced from a lower portion to a magnetic fine wire fillings collecting magnetic particles upwardly and a backwash water is flowed from an upper portion downwardly.

CONSTITUTION: A cooling water containing radioactive iron oxide is introduced into a casing 2 from a stream of a purifying process of the primary cooling water of a nuclear reactor through a valve 20 provided at a lower position of a casing 12 to collect particles such as Fe2O3, or the like contained in the primary cooling water, thereafter a valve 22 is opened to send a gas such as argon, nitrogen or the like into a gas introducing pipe 16, a gas is passed through a rectifying plate 15 and raised as gas bubbles from a lower portion of fillings 14 to easily detach paticles collected in the fillings. Subsequently, a valve 22 and 23 are closed and a valve 24 is opened to introduce a backwash water through an introducing pipe 18 and the residual radioactive particles are washed out from a casing 12 and discharged out of the system through a valve 25.