The revivification of university extension has been regarded as manifestation of the Wisconsin Idea. But the relationship between them has not been made clear. This paper aims to explain (a) what the concept of the Wisconsin Idea was, and (b) how the Wisconsin Idea did affect the revivification of university extension. Findings are as follows : (1) The Wisconsin Idea had three phases ; political reform in Progressivism, University's service to the state government, and University's service to the people. It has changed the emphasis from political dimension to University's service. (2) University's service to the state government meant drafting and advisory work by faculty members of University. (3) University's service to the people has stemmed from the democratization in education. (4) Being supported out of public fund, the University has a duty to perform and cannot shirk it, which underlied university extension.