Resin concrete has many advantages for the structural members. It is of neccessity for resin concrete members to maintain adequate durability under maritime environments.
　The paper reported the experimental results of rosin concrete obtained by the age of five years, in which the additives of resin concrete were three kinds of unsaturated polyester. The specimens were exposed to such conditions as in the natural atomophere, at the tidal zone of the sea water and under the sea water. Testing items were comperssive strength, bending strength, bond strength between resin concrete and PC tendons, and bending strength of resin concrete reinforced with FRP.
　Test results show as follows:
　(1) The tidal zone is the most severe sea condition regarding the deterioration of resin concrete.
　(2) Resin concrete mixed with polyester resin of isophthalate has comparatively higher durability than the others.
　(3) Resin concrete added with silane shows no distinguishable effect.
　(4) Pitting is observed on the surface of PC tendons embedded in resin concrete at the age of five years.