Over-bank flow in a straight channel with a meandering main channel is discussed based on experimental results. The flow structure is characterized by secondary flow and fluid mixing between the main channel and flood plain flow. The secondary flow was directly visualized by a submergible video camera. The secondary flow shows spiral motions and affects the distribution of boundary shear stress and sediment transport. The relation between these structure and the traction process of bed materials is discussed on the basis of the distribution of tractive force visualized by a new method. In the experiments, we used light materials whose critical tractive force is equivalent to the mean shear stress on the bed.