PROBLEM TO BE SOLVED: To provide a device and a method for decontaminating radioactive substances in a water area.SOLUTION: A contaminate water area decontamination device comprises: a closed container 10 which has an introduction section 11 which introduces contaminated water containing radioactive substances, a discharge section 12 which discharges treated water after decontamination treatment and an opening-closing section 13 which can seal the closed container 20; a water permeable container 20 which is detachably stored in the closed container 10; powder or granular absorbent 25 which is filled in the water permeable container 20 and has a dimension larger than a pore size of the water permeable container; a water collection device 30 which is installed on the discharge section of the closed container 10 and discharges the contaminated water after sucking the same; and a submerged pump 40 which is connected to a float 41 and pumps the contaminated water containing the radioactive substances.
