The present paper was concerned with a part of the series of study on the flow in a rough wall channel by authors. For the response of a channel flow to the step change in wall condition, the flow through a two-dimensional channel with short length of rough surface has been investigated numerically. The roughness elements were of the repeated square ribs with the pitch ratio W/h=2 (W and h denote the roughness spacing and height, respectively). The length of rough surface was equivalent to the channel width (corresponding to an ”impulse” of surface roughness for the turbulent boundary layer). The surfaces of upstream and downstream from roughness were smooth and were of sufficient length to allow a fully developed in the smooth wall channel (plane Poiseuille flow) to be established. The Navier-Strokes equations (stream function-vorticity formulation) were solved by the finite difference method using the pseudo-unsteady technique. The calculation was performed for the flow in the different three cases of rough wall channel, and for the range of the Reynolds number Re<1000. From the calculation results, the flow pattern over the rough wall, the behavior of vortex in the roughness groove, the velocity distribution, the pressure distribution along the channel and the distribution of wall shear stress were presented. On the first stage, the particular attention was paid to the comparision of flow structure in the rough wall channel for the different three cases. The results showed that the flow structure varied with the different three cases of rough wall chnnel. The distance of readjustment to the plane Poiseuille flow (relaxation distance) depended on both the geometry of rough wall and the Reynolds number. It was noticeable results that the effects of roughness on the flow extended to the upstream region from surface (its distance being roughly 10 times roughness height for the case the upstanding roughness at Re=100).