This paper presents a case study of a new ten story 83,700 ft² (7780m²) office building in downtown Tokyo with a mix-mode ventilationsystem from the mechanical designer's point of view. Natural ventilation (NV) is an effective method to reduce the energy consumption of buildingsby turning off the operation of air conditioner. This technique has been practiced for decades in Japan but it has rarely been realized at a tenantoffice building in a dense city where multiple tenants occupy the building. In this paper, the design approach of the natural ventilation systemintegrated to the building and its overall system is presented.