In order to apply the logarithmic velocity distribution law to open channel flows with rough beds, it is necessary to define the equivalent roughness and the virtual wall height. In previous studies the equivalent roughness and the virtual wall height have been estimated with a measured velocity distribution. However, when the relative roughness is large, the region where the logarithmic velocity distribution should hold is restricted in a narrow region of a flow. In the flow with large scale relative roughness, therefore, an error of estimation of the virtual wall height from the velocity distribution necessarily becomes large, and the virtual wall height must be determined with the wall shear stress measured directly. From this point of view we measured the wall shear stress directly using a servo typed shear stress meter, and examined the virtual wall height. In spite of the same arrangement of the roughness elements, the virtual wall height depends on the relative depth and the slope of the flume.