Vibration of high-speed trains increases in tunnels by aerodynamic force whose mechanism is unknown. To investigate this aerodynamic force, running tests and a numerical simulation were conducted. Analysis of the running test data showed that large-scale organized structures exist in the space between the tunnel wall and the train. These flow structures develop from the head toward the 6th - 8th cars and become steady thereafter to the tail of the train set. Our computation revealed the mechanism of the generation of these flow structures. The flow becomes unstable by the difference in the velocity profile between the flow under the train and that on the side of the train. The resulted vortices are spread on the train side by the tunnel wall, and then the unsteady aerodynamic force is generated when the vortices pass.