A high-speed train entering a tunnel generates a compression wave ahead of it. The wave is propagated through the tunnel and causes an impulsive noise when it arrives at the tunnel exit. The magnitude of impulsive noise from the tunnel exit is proportional to the maximum time rate of pressure change of the compression wave reaching the exit, according to the aeroacoustic theory. From this point of view, it is important to clarify the formation mechanism and the waveform of the compression wave generated at the entrance of the tunnel. In this paper, the entry compression waves are measured in actual high-speed train/tunnel systems and the results are compared with the theoretical values. Then the formation mechanism of the entry compression waves is clarified.