The aim of this paper is the development of a bridge diagnosis system for daily bridge maintenance. The system includes the technology of wireless sensor network, signal processing, and structure analysis of a bridge. We collect the vibration data caused by external force such as wind pressure, running vehicles and etc through wireless sensor network and then use independent component analysis (ICA) and spectral analysis to analyze the data for extracting character frequency. The diagnosis technology is developed to understand the phenomena like deterioration and corrosion of a bridge through the measurement result. So the diagnosis system can evaluate the health condition of a bridge and conduct a comprehensive deliberation on repair and reinforcement work. We apply this diagnosis system to field experiments at Kokura, Kitakyushu city in Japan and successfully extract the character frequency of a bridge.