PROBLEM TO BE SOLVED: To provide a technique to absorb a radioactive substance, which simplifies pouring/drawing an absorbent into/from a container fastened in a divisible manner, and which removes a radioactive substance and prevents the absorbent from flowing out with an inexpensive treated water collecting device configured by combining multipurpose items.SOLUTION: A device for absorbing a radioactive substance according to the present invention comprises: a container 11 that is fastened in a divisible manner; an inlet-side screen 23, installed in the subsequent part of an inlet of this container 11, for diffusing provided treated water; an absorbent 24, poured into the container 11, for absorbing a radioactive substance from the treated water; a treated water collecting device 25, provided in the container 11, for collecting treated water treated by the absorbent 24; and a connecting pipe 23 for connecting a discharge side of this treated water collecting device 25 and an outlet pipe 32 connected to a tre
