Recently, there have been some studies on reduction of fluctuating fluid forces acting on a bluffbody by controlling a small portion of the separated shear layer from the body. However, the characteristics of the separated shear layer in the vortex-generating region, which are key information to determine an effective way to control the fluid forces, have not been clarified yet. In this study, downstream behavior of small vortices generated in the separated shear layer near the separation point is investigated by computer image processing of flow visualization. In particular, coalescence of the small vortices at the phase when a new vortex is starting to generate and at the phase when the growth of the vortex is in the intermediate stage are investigated in detail. Also the velocity field mapped by LDV measurement is compared with the visualized image at the same phase to clarify the mechanism of the generation of new vortices.