Flow separation can be a major factor in the performance degradation of fluid-based processes, as it can result in several deleterious effects, such as vibration and noise. Recently, an active flow separation control device based on a fluidic oscillator that does not require electric power has been reported. This device is able to generate a sweeping jet over a wide spatial range as well as fluid oscillations, and its internal structure eliminates the need for a drive unit. Furthermore, the development of 3D printing technology has enabled the rapid design of these fluidic oscillators, so these units are expected to have applications in flow separation control. Many studies of flow separation control techniques using a fluidic oscillator have been reported. However, these prior studies all focused on the dynamic forces acting on an airfoil in conjunction with a fluidic oscillator. There have been no studies regarding the flow structure of the sweeping jet emitted by a fluidic oscillator, and the separation flow control mechanism associated with fluidic oscillators is not well understood. The goal of the present study was to examine the flow structure produced by interactions between the sweeping jet from a fluidic oscillator and a primary flow. This was accomplished by performing stereo particle image velocimetry (PIV) measurements of a flow field produced by a sweeping jet to the main flow. These assessments allowed quantitative visualization of the development of the 3D vortex structure resulting from the interaction.