We have investigated interaction-structure in a fish school by using ayu (Plecoglossus altivelis). Free swimming of ayu exhibits various self-similar structures such as speed, swimming distance (revi-walk) and waiting time in isolated and group conditions. We consider these self-similar properties facilitate coherent group behavior smoothly. Furthermore, we also have examined the interaction structure in a small population from two to three (or four) fishes. Underlying interaction structure in collective behavior would make a fish school stabilize and unstabilize throughout one series of events.