A composite forming method, comprising: manufacturing a composite structure body having a hollow region by disposing a rubber-made core having a tubular structure at a portion that forms the hollow region of the composite structure body, the core having a thickness varied in a direction perpendicular to a longitudinal direction of the core to be foldable inward at a thin portion; and pulling the core out of the hollow region by folding the core inward with air being discharged from an inside of the core after the composite structure body is manufactured, wherein the composite structure body is a composite structure body comprising a panel and a reinforcing member having a hat-shaped cross section and attached to the panel or a composite structure body comprising a panel and a reinforcing member having a corrugated cross section and attached to the panel, wherein the composite structure body is manufactured using the core that has a shape that fits a hollow region between the reinforcing member and the panel, and the core has: a large thickness portion that supports, from an inner side of the reinforcing member, a rounded portion of the reinforcing member protruding in a direction away from the panel, wherein the portion that supports the rounded portion reduces or eliminates flexibility in a direction perpendicular to the longitudinal direction of the core, the thin portion that has a smaller thickness than other portions including the large thickness portion in the direction perpendicular to the longitudinal direction of the core, and that is near the panel, and a sharp portion that supports, from an inner side of the reinforcing member, a rounded portion of the reinforcing member protruding toward the panel, the sharp portion being configured to slide over a surface of the panel in a direction perpendicular to the longitudinal direction of the core when the thin portion is folded inward. manufacturing a composite structure body having a hollow region by disposing a rubber-made core having a tubular structure at a portion that forms the hollow region of the composite structure body, the core having a thickness varied in a direction perpendicular to a longitudinal direction of the core to be foldable inward at a thin portion; and pulling the core out of the hollow region by folding the core inward with air being discharged from an inside of the core after the composite structure body is manufactured, wherein the composite structure body is a composite structure body comprising a panel and a reinforcing member having a hat-shaped cross section and attached to the panel or a composite structure body comprising a panel and a reinforcing member having a corrugated cross section and attached to the panel, wherein the composite structure body is manufactured using the core that has a shape that fits a hollow region between the reinforcing member and the panel, and the core has: a large thickness portion that supports, from an inner side of the reinforcing member, a rounded portion of the reinforcing member protruding in a direction away from the panel, wherein the portion that supports the rounded portion reduces or eliminates flexibility in a direction perpendicular to the longitudinal direction of the core, the thin portion that has a smaller thickness than other portions including the large thickness portion in the direction perpendicular to the longitudinal direction of the core, and that is near the panel, and a sharp portion that supports, from an inner side of the reinforcing member, a rounded portion of the reinforcing member protruding toward the panel, the sharp portion being configured to slide over a surface of the panel in a direction perpendicular to the longitudinal direction of the core when the thin portion is folded inward. a large thickness portion that supports, from an inner side of the reinforcing member, a rounded portion of the reinforcing member protruding in a direction away from the panel, wherein the portion that supports the rounded portion reduces or eliminates flexibility in a direction perpendicular to the longitudinal direction of the core, the thin portion that has a smaller thickness than other portions including the large thickness portion in the direction perpendicular to the longitudinal direction of the core, and that is near the panel, and a sharp portion that supports, from an inner side of the reinforcing member, a rounded portion of the reinforcing member protruding toward the panel, the sharp portion being configured to slide over a surface of the panel in a direction perpendicular to the longitudinal direction of the core when the thin portion is folded inward.