In the present study, an ellipsoid reflector was designed and produced using silicon resin in order to generate and focus expansion wave in water for the purpose of a medical application. Across an expansion wave the static pressure, temperature and density decrease, as a result, negative pressure is given behind the expansion wave generated in the water, and then a tensile stress is induced. As an acoustic impedance of the silicon resin is almost similar to that of water, the interface of air-silicon resin would be regarded as the same with that of the air-water. A high voltage discharge was used as an energy source of underwater shock wave. When the underwater shock wave reflects on this interface, it is already clear that the shock wave converts to the expansion wave. The generation and focusing of the expansion wave were carried out and the phenomena were observed by flow visualization using a high-speed camera.