Flows around two tandem circular cylinders in a stream up to the range of much higher Reynolds numbers than the critical one were investigated in a low speed wind tunnel. By making the surface of the upstream circular cylinder rough with two types of surface roughness, the critical Reynolds number of the cylinders was reduced. Drag coefficients and Strouhal numbers of the two tandem circular cylinders were measured and the flow patterns on the cylinders were visualized by the surface oil-flow technique, in the subcritical flow regime, in the supercritical one where there were laminar bubbles followed by turbulent separation and in the transcritical one where purely turbulent separation occurred without laminar bubbles but with an extremely regular vortex shedding behind the cylinders. The flow characteristics of the two tandem circular cylinders were discussed and the effects of Reynolds numbers and the gap spacing between the two cylinders were made clear.