A micro-pressure wave depends on the pressure gradient of the compression wave at the tunnel exit. We measured and investigated the distortion processes of the compression waves during propagation in a Shinkansen long slab-track tunnel. The results confirm that the degree of steepening of the compression wave depends on not only the pressure gradient but also the waveform itself. For this reason, the optimum initial waveform of the compression wave at the tunnel entrance, which is produced by the effective window pattern of a tunnel entrance hood, should be determined considering the distortion process of the compression wave.