It has been said that there is no stable and permanent social unit in a society of chimpanzees. From observations at Filabanga in Western Tanzania and some other observed cases, we concluded that a large-sized group consisting of 30–50 animals is a social unit common to chimpanzees, and pointed out that chimpanzees freely repeat grouping and dispersion within a large-sized group, and such changeability in grouping is the most characteristic nature of chimpanzee society. The size, composition, and various other characteristics of the large-sized group are discussed and compared with the group of Japanese monkey and gorilla. A large-sized group of chimpanzees is not a group basically consisting of a one-male group like a group of gorilla, but usually contains more than five adult males and more than ten adult females as its members, and when they gather to form a larger group, young adult males rarely join it. In a largesized group there is no stable social unit lower than the group itself. It may be considered that two or three large-sized groups concentrate in an area and form a community, but this problem and inter-large-sized group relations are important subjects to be solved in the future. We related these various characteristics of the social organization of chimpanzees to the developed behavior displayed by the chimpanzees, especially those living in the savanna woodland, and discussed one important stage in the evolution of society in non-human primates.