The design service life of submerged tunnel, which is one of the important facilities in port area, is rather long, 50 to 100 years. As for long-term durability of concrete and steel, composing main structures of submerged tunnel, essential knowledge has been obtained in terms of deterioration mechanism and countermeasure for life extension, based on results of the previous intensive studies. However, long-term behavior of rubber, which is common as a main constituent of joints of submerged tunnel, is not fully made clear, because rubber is generally used as consumable products with rather short service life. Besides, rubber used in the joints suffers large deformation and high stresses, causing severe conditions to the material. For ensuring safety of submerged tunnels throughout long service life, it is required to make clear long-term durability of rubber under marine environments.
　This paper describes results of experiments on investigating time-dependent deformation characteristics of rubber as a constituent of joints of submerged tunnel. Based on the results of creep and relaxation tests on rubber specimens, structural safety of the joints is examined. Furthermore, long-term behavior of rubber exposed to seawater is investigated.