A first laminated body is worked to form a second laminated body, the first laminated body being formed by laminating prepregs including reinforcing fibers and resin, the second laminated body including a flat portion and a wavy portion, the flat portion being located at at least one of side edge portions of the second laminated body, the wavy portion being located at a portion adjacent to the side edge portion and extending along a longitudinal direction. An intermediate product is formed from the second laminated body by a forming die such that the flat portion becomes a bent portion that is an inside portion whose circumferential length is shorter in a curved portion, and the wavy portion becomes a bent portion that is an outside portion whose circumferential length is longer in the curved portion.