The purpose of this study is to show interference around two flat plates with a tandem arrangement in uniform flow. This type of flow is one which is seen as basic arrangement of compact heat exchanger. Generally speaking, as for the blunt bodies, pressure drag is large and the numerical value depends on back pressure of a flat plate. By change of the spacing between two flat plates, it is possible to control the back pressure of upstream plate. Using Flow visualization, we measured the reattachment point length for the spacing between two flat plates. It could be obtained from the flow visualization experiment that the reattachment point length was not increase in a continuous manner as two plates spacing length increase.