Using Pressure Sensitive Paint, the pressure distributions on thin wings of the Busemann biplane model were measured in the small supersonic indraft wind tunnel. The observed phenomenon is so complicated that it is completely different from the prediction of the simple 2-D theory. Referring to Schlieren picture and CFD analysis, it is found that these complex pressure fields are caused by the shock wave generated downstream just behind the wing ridge line. The results of this experiment show that the interference between Busemann biplane wings is sensitive to a small change in flow condition. A care must be taken in a wind tunnel test to realize the design condition of Busemann biplane where the shock waves are completely cancelled with each other.