For prediction of the micro-pressure wave, which is one of the environmental problems in high-speed railways, it is necessary to clarify the distortion mechanism of the compression wave propagating through a tunnel. In this study, field measurement and numerical simulation were performed to investigate the distortion of the compression wave generated by a train entering a tunnel as it propagated through the tunnel. The field measurement was performed in a Shinkansen slab-tracked tunnel of 26 km in length with side branches, which was the longest mountain tunnel in the world as of 2004. The pressure of the compression wave was measured at 12 locations in the tunnel. In the numerical simulation, the distortion of the compression waveform was calculated by one-dimensional compressible flow analysis taking into account steady and unsteady friction on the tunnel wall and by acoustic analysis on the effect of side branches. The results of the numerical simulation agreed well with those from the field measurement and clarify the distortion process of the compression wave during its propagation in the tunnel with slab tracks.