Analysis and model experiment are performed on the compression wave generated by a train entering a tunnel with an entrance hood, which is a prevailing countermeasure to reduce the micro-pressure wave emitted from a Shinkansen tunnel portal. The tunnel entrance hood is a structure to extend a tunnel and has openings at its side walls or roof to decrease the pressure gradient of the compression wave generated by train entry. The hood design has principally been performed through model tests since three-dimensionality of flow field is generally important. Thus, a more efficient design method is required. In this paper, an analytical method based on the aeroacoustic theory developed by Howe is applied to a short “acoustically compact” hood having a window on its side wall. The results are in good agreement with experiments including the train offset effect in a double-tracked tunnel. It is also clarified by the analysis that the performance of the entrance hood is greatly influenced by the relative length of the train nose to the distance between the window and the hood entrance.