In this paper, in order to investigate the mechanism of pseudo-shock waves and the effect of the inner diameters of pipes on a jet, experiments for the underexpanded sonic jet issuing from 5 kinds of pipes were carried out, mainly for mean Mach numbers ranging from 1.7 to 2, at the pipe exit. So the flow visualization and the pressure distributions were studied in detail. The flow visualization was photographed by both the schlieren and shadowgraph methods. When a Pitot-tube was placed in a supersonic flow, a curved shock wave was apt to be formed ahead of the tube. Therefore, in order to prevent its growth, a static pressure probe with a conical heat was used in this study. As a result, the general mechanism and length of pseudo-shock waves, the pressure distributions in a slip surface, and further, the effect of the inner diameters of pipes on the characteristics of an underexpanded jet were clarified.