Yoshii Locks, being a prefecturally protected historic landmark, are the oldest remains in Japan, although its value has not been thoroughly acknowledged nationwide. The reason is the third old Minuma Tusenbori Canal Locks in Saitama Prefecture has been too excessively advertised as the oldest based on the wrong data, and also they has been located close to the Capital. In this paper, the fact that Yoshii Locks are the oldest remains is proved from the viewpoints of historiographical materials, nationwide survey of sluices and also authenticity of the locks themselves. That is to say, the historical value of the locks is described by combining their planner, Nagatada Tsuda who was a “gundai” of the fief of Okayama. And the authenticity of the locks is checked by the detailed examination of stonework.