Diffusion of Cl- ion in hardened cement paste with slag and siliceous powder such as silica fume and white carbon was investigated. The addition of admixtures reduces the content of Ca(OH)2, which is the main cause of pore formation by corrosive action of sea-water. The addition of admixtures makes the hardened cement paste dense, thereby restricting the diffusion of Cl- ion and improving the resistance to sea-water. Apparence diffusion coefficient of Cl- ion in hardened ordinary portland cement paste was 3.7${ imes}$10-8$ extrm{cm}^2$/sec, while that for the hardened cement paste with the admixture was 1.2∼3.2${ imes}$10-8$ extrm{cm}^2$/sec.