A rotary body used in an energy storage device and capable of storing a large amount of external energy is provided in an energy storage device that stores external energy as the energy of the rotary motion of the rotary body for which the frictional resistance of the bearing parts has been reduced to a high degree using the fishing effect of superconductivity. The rotary body is used in an energy storage device capable of storing energy by rotating the rotary body for which the frictional resistance of the bearing parts having floating support that makes use of the fishing effect of superconductivity has been made very small. The rotary body is made of CFRP, and the required compressive stress is applied to it in the direction opposite to the centrifugal force of the rotary body when rotating. The rotary body has a bar-like structure elongated in the direction of the centrifugal force when the rotary body rotates.