A number of volunteer groups, which sprang up after the Great Hanshin Earthquake to help victims of the killer earthquake, were sustained not only by spontaneous volunteers who rushed there individually but by members of established organizations. They came from (a) industrial organizations, (b) semi-governmental non-profit foundations, (c) non-governmental organizations, and (d) associations. To clarify the backgrounds of the movement, intensive interviews were conducted with participants in the organized relief activites. The results showed that changes in Japanese organizational characteristics in the 80s triggered the dramatic rise of volunteerism and growing interest in it in the 90s. People from those organizations cooperated with each other over the established boundaries between them in the emergency because they are less tightly linked to their organizations than before. The results also indicated that the relationships between Japanese volunteer groups and the two major established organizations (i. e., government offices and private companies) can be classified into five categories: (a) have no connection with them, (b) support their functions, (c) be a third, independent sector, (d) substitute for them temporarily in an emergency, and (e) coordinate with them in a network. While (a) and (b) were dominant in the 70s and 80s, (e) will become popular in the 90s.