The goal of this study is to slarify the mechanizum of forming up the down town office area in Tokyou. Forcusing at the rental office market, and through analysis of key factors in the rental office market, this study determined the five elements which consists of office floor stock, density of office in the area, rent, increace of office floor and distance from the business center as main facters. It is also found out that down town Tokyo office area is composed of five characteristic office zones, and that the structure of the market is a strong one center market. Since late 1980's Marunouchi and its neighborhood area attracted most of the new supply and development method with bonus were heavy used in the supply process.