To easily manufacture a lightweight, high strength and high rigidity FRP structure at a low cost by providing a resin diffusion means for a core with FRP reinforcing members arranged discontinuously any integrating reinforcing fiber bases and the core for an FRP plate with a resin.

Reinforcing bases 23 for forming FRP reinforcing members having parts extending perpendicularly with an FRP plate after molding are discontinuously arranged on one of the end parts of cores 22. Inside a mold 21, the cores 22 consisting of foams or the like are arranged and the reinforcing fiber bases 23 are arranged on at least both faces of the cores 22, each of which has a larger groove as a resin passage and many small grooves branching from the large groove. The resin is diffused in the planar direction of the reinforcing fiber bases 23 through the large groove and the small grooves and the reinforcing fiber bases 23 are impregnated with the resin thus diffused in the thickness direction of the base 23. Finally the impregnated resin is cured at normal temperature or by heating to obtain the completed FRP structure.