To mold an FRP structure at a low cost and efficiently as a whole in one- shot molding process by impregnating a reinforcing fiber base material with resin in the thickness direction while the resin is diffused in the planer direction of the reinforcing fiber base material through a groove molded simultaneously with a core.

A core material 22 made of a foam or the like is arranged inside a mold 21 and a reinforcing fiber base material 23 is arranged on at least both faces of the core material 22. The core material 22 is formed of plural split parts and a plurality of the split parts of the core 22 are arranged lengthwise and breadthwise on the plane. In addition, the core material 22 has a large groove as a passage for resin and many small grooves branching out of the large groove. A resin is diffused in the face direction of the reinforcing fiber base material 23 through the large groove and the small grooves and at the same time, the core material 23 is impregnated with the resin in the thickness direction of the resin-diffused reinforcing fiber base 23. The grooves may be formed by machine-working as an after-machining process but in this case, the grooves are formed by molding substantially at the same time with the molding of the core material body.