The O-Torii or Great Gate, of the Itsukushima Shrine, Hiroshima prefecture, which is registered as an Important Cultural Property, was first constructed in 1168. The wooden pillars of the Great Gate are submerged in seawater up to about three meters from the base at high tide. These pillars are made of unworked camphor wood with bases measuring about 3.6 meters in diameter. The Great Gate has been restored eight times in the last 800 years, with the most recent restoration work carried out in 1950. Since the giant camphor trees are almost impossible to find nowadays, the Great Gate must be preserved as long as possible. When the authors investigated the biodeterioration of the Gate in August 1981, the parts of the wooden pillars submerged in the sea at high tide had been eaten away by about ten centimeters all around, and the present surfaces were covered by several kinds of marine organisms. The inside of the pillars, however, were not yet damaged by marine borers. The authors collected and identified some marine borers from the Great Gate and also discussed the morphology, ecology, and distribution of these marine borers. The authors studied two marine borer control methods: treatment with an antifouling agent and a fumigation treatment. -- AATA