The development of the Karman vortex street due to natural convection in the opposing flow was discovered and has been clarified by means of the numerical simulation by Noto & Matsumoto. A flow visualization is carried out to verify the numerical prediction on the development of the Karman vortex street. As a result, the numerical simulation results are in good agreement with the flow visualization results. The validity of development of the Karman vortex street due to natural convection is verified by flow visualization. The flow without vortices is really varied to the Karman vortex street, the large vortex street and the natural convective vortex by natural convection as predicted by numerical simulations.