Due to their higher bridge piers and massive girder loads, long span bridges over rivers tend to have a longer natural period of equivalence. Therefore, during seismicity, train-running quality is impaired on the foregoing bridges. In this study, the authors analyzed numerically dynamic behavior of the train-running quality on a long span bridge, and clarified the following items: Firstly, the train-running quality on the bridge becomes lower on girders in the middle of the bridge. Secondly, folding displacement at edges of the bridge did not largely affect the train-running quality. In addition, the authors studied on improvement measures by taking rigidity of piers and bearing as a parameter, and clarified that the train-running quality be improved with provisions of a large-size caisson or a sliding bearing.