Vibration of high-speed trains increases in tunnels caused by aerodynamic force whose mechanism is unknown. To investigate this aerodynamic force, the authors conduct an analysis of the running test data and a numerical simulation. The running test data indicates that the aerodynamic force acts to vibrate the train in the tunnel and it originates from large-scale coherent structures in the space between the tunnel wall and the train. These flow structures develop from the head toward the 6th to 8th cars, and become steady thereafter to the tail of the train set. The computation reveals that the flow becomes unstable under the train. The resulted vortices are spread on the train side by the tunnel wall, and then the unsteady aerodynamic force arises when the vortices pass.