In IT technology, especially in software development, open development system has been proven to be useful. Linux, apache software and GNU software are typical examples. Still, it is also known that there are many open software development projects that failed or stopped developments. So far, it has not been clearly known what the key factor is to give the difference between the successful open projects and the ones that fails. In this report, we analyze the sustainability of open systems based on an agent model that explicitly takes the effect of fluctuation into account. Although the model is simple, it is shown that the system with many less-powered leaders is more stable than a system with small number of powerful leaders.