A compression wave is generated ahead of a high-speed train, while entering a tunnel. This compression wave propagates to the tunnel exit and spouts out as a micro pressure wave, causing an exploding sound. In order to estimate the magnitude correctly, the mechanism of the attenuation and distortion of a compression wave propagating along a tunnel must be understood and experimental information on these phenomena is required. An experimental and numerical investigation is carried out to clarify the nonlinear effect of the propagating compression wave in a tube. The attenuation and distortion of compression wavefront are affected by nonlinear effects on the propagating compression wave in a tunnel. It is confirmed that a head of wavefront and an overshoot on pressure distribution is observed behind a compression wave due to the transition of the unsteady boundary layer.