Steelmaking slag came to be used in a marine environment and safety issue of slag material is a matter of concern. We are trying to provide the safety information of the slag material. Experimental facility with mesocosm aquarium tank, which integrated tidal flat area and shallow area, was constructed in order to evaluate the environmental impact. The facility consists of two aquarium tanks, one is the experimental unit using slag materials as dredged soil improver, called CaO-improver, and another is the control unit not using slag materials. Long-term operation over the five years from 2012 to 2017 was carried out to evaluate the long-term safety of slag materials. We observed not only the changes of water quality but also the acute and chronic toxicity assessment using three kinds of aquatic marine organisms such as bioluminescent bacteria, micro algae and copepod for seawater in the aquarium tanks. As a result of the long-term experiment, stabilization of bottom sediment by CaO-improver could be confirmed, but on the other hand, influence of alkali elution from slag was estimated. Furthermore, as a result of three bioassays, it was judged that there was no effect of acute toxicity and chronic toxicity on organisms caused by slag material within the range examined.