The environmental destruction of marine ecosystem caused by micro-organism included in ship ballast water has been a global problem. The International Maritime Organization (IMO) adopted strict standard rule for control and management of ship ballast water in 2004 , after that, many ballast water treatment systems have been proposed and developed all over the world. However, a lot of practical problems have been remaining yet for development of energy-saving and space-saving systems. In general systems, shipping companies have to charge and manage chemicals on board to kill marine bacteria in the ballast water. Therefore, if troubles should happen in the ballast water treatment process, leaking chemicals from the system might contaminate the sea. In order to realize more secure and environmental friendly treatment method, the authors have proposed a new sterilization technique of ship ballast water using underwater shock waves. In the previous research, the result showed that a marine
sp. was completely inactivated when the excess pressure in cell solution contained in a small aluminum container was over 200 MPa in impact experiments by a gas gun . In general, electric discharge, explosive or high-speed collision of a projectile is used as a power source to produce underwater shock waves and has been applied to engineering and medical fields. However, those power sources are unsuitable for practical use on board from a point of view on energy cost and safety. From the above-mentioned, the authors thought of killing marine bacteria in a large amount of ballast water by exposing to strong pressure pulses and free radicals created from collapse of microbubbles [3-6]. In this idea, the excess pressure of underwater shock waves plays only a role of leading to collapse of microbubbles, so that it does not necessarily need to release an extreme high-energy in water. Therefore, the method of hitting water surface with shock waves produced in gas would be an effective one of underwater shock wave generation on board. In addition, it needs to develop the shock wave generator that can produce shock waves periodically by low driving cost for practical use.