PURPOSE: To improve adhesion between carbon fiber and an unsaturated matrix resin, by treating surface-oxidized carbon fiber with an epoxypolybutadiene.

CONSTITUTION: A strong bonding force is produced between carbon fiber and an unsaturated matrix resin, by treating surface-oxidized carbon fiber with an epoxidized polybutadiene. As the unsaturated matrix resin, there can be used any type including an unsaturated polyester resin and a diallyl phthalate resin. In one embodiment, a still higher effect is obtained by using carbon fiber treated with an epoxidized polybutadiene-containing, surface-treating resin composition and employing a composition prepared by adding at least 1% of an epoxidized polybutadiene to an unsaturated matrix resin.