Natural disasters due to local heavy rains and storms have been increasing over and under the ground owing to climate change. Since 2009, NTT DOCOMO, INC. has been working on a nationwide weather observation network, the Environmental Sensor Network, by installing weather observation devices in wireless stations nationwide. In addition, the company has assumed that weather observation data are the trigger factor that may cause disasters and promoted the provision of comprehensive observation data, including river water levels, directly related to disasters such as floods. This paper presents an outline of the environmental sensor network as well as its functions and applications to the disaster prevention field.