In the cement industry, ground granulated blast furnace slag, which is an admixture, is used as an effort to reduce environmental impact. There are various characteristics of ground granulated blast furnace slag which can be useful as a substitute material for cement. However, carbonation resistance is low, and it can be a problem. In this study, it was investigated the effect of carbonation using ground granulated blast furnace slag on water permeability and it was also measured the porosity by Archimedes method and mercury intrusion porosity method. It was investigated the alteration by measuring the pore structure before and after car- bonation of using ground granulated blast furnace slag. As a result, it was found that using blast furnace cement can easily permeate water after carbonation. About the influence of different curing periods, when the curing period was short, it was found that water can easily permeate in the samples. The result measured by Archimedes method showed that the porosity increased when carbonation occurred. As a result of mercury intrusion porosity method, there are many small diameter pores when carbonated, and it has a complicated pore structure. In conclusion, carbonation when using blast-furnace cement is considered to reduce the mass transfer resistance by forming a continuous pore structure.