When a high-speed train enters a tunnel, a compression wave is generated ahead of the train and propagates along the tunnel. This wave subsequently emerges from the exit portal of the tunnel as an impulsive wave which may cause noise problems in the surrounding area. The objective of this paper is to show the optimum silencer to reduce an impulsive noise. The effects of the annular boxes at the circular tube exit on the impulsive noise were investigated numerically. As a result, the effects of geometry of passive silencer and the noise reduction rate have been clarified, and then a suitable choice of design factors for exit anular box has been suggested.