The purpose of this study was to clarify storm water runoff characteristics of the extensive green roofs. The experiments were conducted with the three different water retention drainage, Styrofoam panels (25mm, 30mm and 40mm). In order to compare the data of the traditional, intensive green roof system (Murayama. T. et al, 1997), the experiments measured the peak runoff volume, the runoff coefficient, and the runoff starting time. The results showed that the extensive green roof system could delay the runoff starting time for 25 minutes and peak runoff arrival time for 77 minutes. In addition, they cut the 0.09L runoff volume. The study showed the extensive green roof systems the ability to retain and delay runoff and the potential effects to reduce environmental impacts.