The purpose of the present invention is to provide: a structural material for structures which is capable of attaining reductions in working time and cost in production steps and of preventing an increase in weight; a fuel tank; a main wing; and an aircraft. A rib (11) as the structural material for structures is characterized by comprising a carbon-fiber-reinforced plastic wherein the reinforcement comprises carbon fibers and the matrix comprises a plastic, and the surface of the carbon-fiber-reinforced plastic was coated with a low-viscosity surface-protective material (18) having conductivity imparted thereto.