The removed soil generated from decontamination projects in Fukushima Prefecture is being transported to the interim storage facility (ISF) at present. In the ISF, modification agent such as water-absorbing polymer is used to improve handling of the removed soil during the separation process before storage. The expansion characteristic of the water-absorbing polymer agent might change the hydraulic property of the soil which will affect the leaching behavior. In addition, degradation of organic matter contained in the removed soil might influence the leachate quality as well. Thus, it is necessary to evaluate the environmental safety of the modified removed soil for safe storage in the ISF and its potential reuse in the future. In this study, two lysimeter tests on modified and unmodified removed soil was conducted to investigate its leachate quality. These tests lasted more than 400 days (liquid/solid ratios greater than 3.5). The highest 137Cs concentration in leachate from the unmodified soil was 6.6 Bq/L at 250 days, and the accumulated leaching ratio was 0.09%. In leachate from the modified soil (addition of 3% water-absorbing polymer agent powder), the highest 137Cs concentration was 3.3 Bq/L and the leaching ratio was 0.046%, a reduction by half. Leaching of 134Cs from both soils was extremely low. The soil modification resulted in accumulated leaching of total organic carbon that was 60 % higher and leaching of K+ and ammonia nitrogen (NH4-N) that was 35 % lower, respectively. No expansion of soil volume was observed after modification.