When the underexpanded supersonic jet impinges on the obstacle, it is well known that the self-induced flow oscillation occurs under the specific condition, namely the pressure ratio in the flowfield, the position of an obstacle and so on. This oscillation is related with the noise problems of aeronautical and other industrial engineering so that the characteristics and the mechanism of self-induced flow oscillation have to be cleared to control the various noise problems. This paper aims to clear the characteristics of oscillation frequency, pressure fluctuation on the surface of a cylindrical body through experiments. From the experimental results, it is clear that the Strouhal number of the Mach disk and standoff shock wave depend on the size and position of the obstacle.