This was the first phase of a redevelopment project at Kyorin University Mitaka Campus, that contains both hospital and university. A new gymnasium “Forest Arena” was planned to harmonize with the nearby rich natural environment on an adjacent new site. The gymnasium can also be used as an auditorium. A plant-generating algorithm was used to create the motif in the 43.2-meter-long span frame that surrounds the seats. The arch height, steel frame dimensions and weight of the frame were optimized through computational design, enabling the creation of a space covered by this tree-shaped frame.