An underexpanded jet is one of supersonic jets. The jet becomes underexpanded when the pressure ratio exceeds the critical value across the convergent nozzle discharging it. The jet is not uniform because of the presence of the expansion wave, the compression wave and the shock wave formed in it. In the jet boundary, many vortices are generated by the shearing stress between the supersonic flow of jet and still atmosphere. The interaction of vortices and jet is closely related to the high frequency sound radiating from such flow field. The high frequency sound is called Screech Tone. This Screech Tone has been studied extensively and its relationship to the vortex has been discussed. However, measurement methods are still not established for vortex moving in the vicinity of the jet boundary. In this study, the vortex moving in the vicinity of the jet was experimentally examined. The device consisting of a laser and an optical sensor was applied to detect the vortex. Installing those devices at several points around the jet, the frequency and the time differences of signals between points were investigated. The flow field was visualized using optical methods. And the high frequency sound emitted from the jet is measured using a microphone. As a result, the frequencies of the vortex generation and Screech tone showed good agreement. The vortex frequencies at several locations around the jet were almost the same, indicating that the section-averaged advection velocities of the vortices were almost the same.