The study investigated the relationship between the social capital of farming and fishing groups and the psychological well-being of individual farmers/fishers, as well as the roles of extension officers who promote the social capital of farmers/fishers.   The questionnaire survey conducted among leaders of farming and fishing groups showed that relationship of trust among group members was positively associated with the subjective well-being of both the leaders and group members. Additionally, the results suggested that the extension officers’ assistance in building collaborative organizations among farmers/fishers was effective in solving problems, especially for farming and aquaculture groups. The roles of extension officers, who are a “third party” for the groups, are discussed.