The East/Japan Sea has undergone dramatic changes during the last 50--60 years as the result of a shift in its ventilation system from bottom water formation to intermediate water formation. A moving-boundary box model (MBBM) was developed to quantify these changes. The model consists of three boxes for deep waters representing Central, Deep, and Bottom Waters, respectively, and two boundaries in between, which are moving according to the volume changes of these water masses. The model calibrated with CFC-11 and tritium distribution shows the bottom water formation of ~0.02 Sv in magnitude in the past slowed down and came to a complete halt in mid-1980s and late-1990s. Furthermore, this model predicts the Bottom Water will disappear completely in 2040 as Central Water penetrates deeper. Considering the recent observations that many oceans are in change, a MBBM could be a valuable tool to investigate the nature of such changes.