A sweeping jet produced by a fluidic oscillator has recently been examined from the viewpoint of flow separation control. Studies have proposed active flow control techniques for an airfoil based on sweeping jets. Most of these studies focused on the characteristics of the dynamic forces acting on the airfoil. However, the flow fields formed by the interaction between the sweeping jet and the main flow and their flow mechanisms are poorly understood. In particular, the vortex structure and flow mechanism formed by the interaction with respect to the velocity ratio between the sweeping jet and the main flow and the growth mechanisms of the vortex structure formed by the interaction have not been clarified. The detailed vortex structure generated by the interaction between the sweeping jet and the main flow was quantitatively examined using stereo PIV. The stream tube generated by the interaction between the sweeping jet and the main flow consists of two vortex tubes near the outlet of the sweeping jet at a location of 1.0d to 11.3d. At locations of 22.5d and 30.0d, a pair of vortex tubes gradually separates. Each vortex tube develops in the wake and its vorticity weakens.