It is well known that shock wave formed in an exhaust pipe of a reciprocating engine generates very loud noise when it is discharged from the pipe end into atmosphere. However, there are few reports that describe flow field formed after discharge of the shock wave. In the flow field vortex ring and underexpanded jet appear behind the shock wave. Also, it is possible that vortices moving along shear layer of the jet interact with the shock wave formed in the vortex ring which is generated by so called 'convergent divergent nozzle effect'. The interaction may become noise source. In the present study, we describe the jet structure and the process of the noise generation experimentally or numerically. In the experiment pressure measurement in a pipe, visualization, sound pressure measurement and fluctuating static measurement of the jet are made. In the numerical study we assumed that flow through the pipe should be 1 dimensional and it was solved by using 3rd order MUSCL TVD scheme. Moreover, for flow field generated downstream of the pipe end, 5th order MUSCL TVD scheme is employed in order to capture sound wave.