It has been conducted marine environment improvement tests in Innoshima, the Seto Inland Sea, utilizing slag, byproduct of iron and steelmaking processes. The test aims to improve coastal sea area using 100% recycled materials. It was used steelmaking slag block as seaweed reef, granulate blast furnace slag for sand capping, and steelmaking slag in the shape of stone as submerged dike materials. Steelmaking slag block is a large carbonic solid produced by blowing gas containing carbon dioxide (CO2) into the pulverized steelmaking slag. Results showed that Sargassum and other seaweed were covered on the blocks and species of seaweed were increased as time goes on. These suggest that carbonated slag blocks have good potential as seaweed bed and are applicable to the marine environmental improvement projects.