A gymnasium consisting of two large and small domes, a main arena and a sub arena, like small and large islets floating in the Seto Inland Sea. A spherical dome was placed at the entrance of the twin dome so that the main entrance of the arena on the second floor can be approached along the slope. This spherical dome also serves as an iconic entrance pavilion to the twin dome, such as an observatory, an information facility, and a restroom. The structure of the dome is made of carbon fiber reinforced plastic (CFRP) to take advantage of the world‘s largest carbon fiber factory existing in Shikoku and to reduce the construction time and construction costs by using a lightweight structure. Another reason for the choice of CFRP is an inspiration by Japanese traditional handball “Sanuki-Kagari-Temari” and by the name “Kinushima” (which means “silk island”), seen from the site. Light and white material made of strong thread, form the structure of this new gym.