Bridge deterioration and damage cases have been increasing, and the progression of damage can significantly reduce bridges’ soundness and remaining service life. Therefore, it is crucial to identify the damage’s causes and take countermeasures in its early stages. Fatigue cracks occurred at the joints between the transverse beams and the vertical members of the stiffening truss girders at several parts. On the other hand, the bridge is susceptible to strong winds, such as typhoons in summer and monsoons in winter, and the wind-induced vibration of the entire bridge and its members. In this study, a measurement environment was constructed on the bridge to investigate the cause of crack initiation. Short-term measurements analyzed stress fluctuations near cracks under traffic loads and continuous observations over a certain period under normal service conditions. Short-term measurements confirmed the occurrence of stress fluctuations near cracks in the web connection plates and flange attachments at the ends of the transverse girders that exceeded the assumed endurance limit. We also predicted fatigue life by tabulating the frequency of onset stress fluctuation during the continuous observation period. Then, to clarify the relationship between bridge vibration and fatigue damage, we newly developed a measurement environment to examine the relationship between the damage and bridge vibration by understanding the occurrence of bridge vibration and the stress fluctuations during bridge vibration. In addition, we analyzed the relationship between wind conditions and bridge vibration using wind condition data from meteorological stations in the target area and confirmed whether bridge vibration occurred under high wind conditions during the measurement period. We also analyzed stress fluctuations during relatively large bridge vibrations and examined their involvement in damage due to wind action.