To evaluate the effectiveness of soft CaO-improved soil as tidal flat and shallow sediment material, we conducted a comparative experiment using mesocosm tanks. The soft CaO-improved soil is composed of dredged soil and converter steelmaking slag, which is a byproduct of steelmaking. It is a mixed soil that has been improved by adjusting the particle size and the percentage of the converter steelmaking slag to enable benthic organisms to live in it. In this study, a two-year long-term evaluation using a tidal flat/shallow integrated mesocosm was conducted before application to actual coastal areas. In each experimental tank, soft CaO-improved soil was laid as an experimental site, and dredged soil was laid as the control site. The macrobenthos was observed in the sediments of both the experimental and control sites. More eelgrasses grew in the experimental site, where the light environment was improved as a result of stabilizing the sediment, than in the control site. These results show the utility of the soft CaO-improved soil as the sediment for eelgrass beds and benthic organisms.