A core according to one aspect of the present disclosure is a core that is inserted into a space between a skin and a stringer in a step of integrally molding the skin and the stringer, the skin including fiber-reinforced resin, the stringer having a hat-shaped section that is open toward the skin. The core includes: a first die that extends along a longitudinal direction of the stringer and contacts the skin; a second die that extends along the longitudinal direction of the stringer, is adjacent to the first die, and contacts the skin; and a third die that extends along the longitudinal direction of the stringer, is located at an opposite side of the skin across the first die and the second die, and contacts both the first die and the second die.