PROBLEM TO BE SOLVED: To propose a new method for determining replacement time of a radioactive substance adsorbent, that is capable of properly determining replacement time of a radioactive substance adsorbent.SOLUTION: A method for determining replacement time of a radioactive substance adsorbent is provided for a decontamination treatment apparatus for radioactive contaminated water that includes: a radioactive substance adsorbent filling part which is filled with a radioactive substance adsorbent; and a configuration for allowing water conduction of treated water in a specified direction inside the radioactive substance adsorbent filling part in order to decontaminate the treated water. The method for determining replacement time of the radioactive substance adsorbent includes the steps of: measuring a radiation surface dose rate at a position at an appropriate interval in a water conduction direction of the treated water, in an outer peripheral part of the radioactive substance adsorbent filling part; an
