In this paper I reviewed the studies of rumor published in Japan and abroad. The conditions of occurrence and spread of rumor, transformation of content of rumor, and the life cycle of rumor were considered. Rumor occurs when people are set in ambiguous situation. Psychologists assert that rumor is the projection of the motive or the sentiment of the people. Sociologists assent that rumor is the product of collective behavior, the interpretation of the situation by the people. But it is necessary to integrate these two approachs. It is often said that rumor is false information. But, if it is false, why people believe and communicate it to others? I think rumor is not false information, but necessary one. It presents reality which is different from those presented by the mass media. Rumor is unverified or unofficial information. When people need information and if they are not given official information, rumor occurs. And it also occurs when people don't believe official information. Rumor is the oldest medium of the mankind and it can not be rooted out as long as people talk with each other.