The purpose of this study is to identify the positive aspects of Malcolm Knowles’s idea of adult education from the formative days of this idea. Knowles is famous for his elaboration of the idea of andragogy, the art and science of helping adults learn, but little is known concerning his ideas of adult education in their formative days. In the 1950s, Knowles’s idea of adult education revolved around the idea of informal adult education, together with the theoretical help of Eduard Lindeman and Harry Overstreet. The author recognized that the essence of Knowles’s idea of informal education was to create the maturity of people and a mature society through adult learning. Knowles’s functional approach to adult education is applicable to the ideas and practices of contemporary social education in Japan.