An impulsive pressure wave which is called a "micro-pressure wave" radiating from a tunnel exit is one of the important environmental problems in high speed railways. The magnitude of the impulsive wave depends on the waveform of compression wave at the tunnel exit; the steeper the wavefront of the compression wave, the larger the impulsive wave. In our previous study, the distortion of compression wave during its propagation through a slab track tunnel was investigated by one dimensional numerical analysis which took account of steady and unsteady wall friction and of heat transfer to the tunnel wall. As a result, it is shown that verification of effects of short side branches arranged at side walls of tunnel is needed to predict the distorsion of waveform. In this study, the effects of short side branches are analyzed by acoustic analysis and included in numerical analysis. The results show that the numerically obtained waveforms agree well with field measurements which are obtained from a slab track tunnel of Shinkansen.