When a high-speed train enters a tunnel, a compression wave is generated and propagates to the tunnel exit. At this moment, an impulsive pressure wave (micro-pressure wave) is emitted out of the exit. If its magnitude is large, it poses environmental problems. The magnitude of the micro-pressure wave is closely related to the pressure gradient of the compression wave arriving at the tunnel exit. Hence, to estimate the magnitude of the micro-pressure wave, it is required to investigate distortion of the compression wave propagating through tunnels. In this study, we investigate the distortion of the compression wave propagating through a slab track tunnel with short side branches by field measurement, numerical analysis and model experiment.