In this paper, the behavior of fish school is considered as an autonomous decentralized system, that is, each individual fish behaves autonomously while keeping a specific order based on the information from other fish. The model of such a system consists of a propulsive mechanism, an environmental effect and a schooling mechanism. In our earlier papers, it was shown that fish can make a school if each individual in the school gets the information from only a few individuals around him. In this paper, fish behavior is investigated for the cases where the school is affected by various environments, i. e., approach of an enemy, appearance of an obstacle, and so on. Especially, the relation-ship between the fish behavior and the quantity of information is investigated to analyze the state of each individual in the school.