To provide a resin composition for a retrofitted anchor, which is small in cure shrinkage and exerts excellent adhesive power.

The resin composition for the retrofitted anchor is for use when the anchor member is inserted into a hole bored in an existing member and fixed to the hole, and the resin composition does not contain a hardener component. Specifically the resin composition contains therein one or more urethane modified radical polymerized oligomers selected from the group consisting of a urethane modified epoxy-acrylate, a urethane modified unsaturated polyester, and a urethane modified polyester acrylate, and a radical polymerized monomer.