As the tourists to Kyushu island from China and Korea had increased, Nagasaki city decided to have a cruise ship terminal as a gateway. Since the building was planned not only for the functional management of custom, immigration and quarantine but also for other public events, they required a spacious and pleasing open space about 1000m2. In contrary the building hight is strictly limited and the roof had to be covered with green, since the whole area was designed as a park. To solve the contradiction, we designed the roof as thin as possible by using flat slab system supported by many independent columns, which is also used for sky lights, lighting equipments and air-conditioning. We lifted up the surface of the park and put the traffic networks underneath. Thus the view from the main hall to the sea and the secured observation deck on the roof were realized.