It was shown by theoretical and experimental studies that high temperature and high pressure state is provided in the converging center by converging shock wave or imploding detonation. In particular, the imploding detonation can implode more effectively than the case of the shock wave. The authors examined the microorganism treatment technique using ultra-high pressure underwater shock waves in a steel pipe by the gas imploding detonation until now. The authors tried to confirm the effect of underwater shock wave generation by change minimum converging radial distance of imploding detonation. As a result, the maximum pressure increased in a decrease in the minimum converging radial distance. Therefore, there was large percent of mortality rate with an increase in minimum converging radial distance.