Fish robots that swim with the tail fin do not give the caution to underwater creatures. Therefore, they become useful for the investigation of ecosystem in water and the usage of the environmental surveillance and so on. However, the propulsive velocity of fish robots is extremely slower than that of live fishes. It is necessary to examine the difference of how to swim between live fishes and fish robots in order to put the robots to practical use. In this study, the modification has been added to the CFD code that had been developed up to now, so as to clarify the relation between tail fin's flexibility and the propulsive performance of the fish robot. The new code can calculate fluid-structure interaction analysis considering elastic deformation of tail fin. By using the modified CFD code, effects of tail fin flexibility on propulsive velocity and flow pattern of water around the fish robot have been examined.