A study on the low Reynolds number flow such as the flow in or around the micro device is strongly required along with the development of the micro manufacturing technology. The low Reynolds number flow over a backward facing step is selected as one of the representative examples of the vortex dominant flows in the present study, because the mixing promotion is expected by an oscillatory motion of the vortex in the separating and reattaching shear layer over the step. It is important to clarify the flow fields in small channel or around the small device by flow visualization, since minimum disturbance in the measurement is achieved due to non-intrusive method. The objective of the present study is to clarify the flow behavior in the cross section in the spanwise, transverse and streamwise direction by the flow visualization using a high speed video camera. The Reynolds number based on the step height and the bulk velocity is set at 380 to 960. The visualization results in the cross section in the spanwise direction show that the separating shear layer from the step edge introduces a series of the primary vortices which have a rotation axis around the spanwise direction, and the main stream has a regularly whipping, wavy motion caused by the vortices moving toward the downstream direction along the upper and lower walls. The observation in the cross section in the transverse direction indicates that a scale of the vortex length in the streamwise direction is almost constant, but the primary vortex shows a periodic change in the spanwise direction, as the Reynolds number increases.