In 2020, the autonomous underwater vehicle (AUV) has already become a vital part of deep-sea research. There is a long history of R&D of AUVs that dive into the deep sea, where radio waves cannot reach, thus making remote control difficult so that no help can be provided, which implies that careful and adequate preparation is necessary. Their successful development has been based on the accumulation of experience and achievements contributing to the remarkable results that no other system can produce. The aggressive R&D of Japanese AUVs started approximately 40 years ago. This paper looks back at this history and introduces various Japanese AUVs.