In this study, we conduct fundamental study of structural health monitoring based on changes of modal amplitude. The free damped vibration of a pedestrian bridge is measured by using 3D wireless acceleration sensors. The pedestrian bridge has local damage on its girder. The vibration measurement is repeated after the repair work of the girder. From measurement results, natural frequencies of each mode don't change by repair work. Modal amplitudes are changed in several vibration modes. COMAC method is applied to discuss the effect of modal amplitude changes and the change of COMAC is larger on the nearest point of the damage.