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To elucidate the effects of hydrographic conditions on the migration and distribution of diamond squid (Thysanoteuthis rhombus) in the Sea of Japan, two numerical experiments were conducted using a Lagrangian particle-tracking model. First, Lagrangian simulations with different horizontal swimming speeds were conducted in the area off the San-in coast, the southwest Sea of Japan. The comparison results of simulations and experimental tagging records from 2003 suggest that horizontal movement of diamond squid over a few days is influenced by individual swimming, but long-term migration over more than about 10 days is dependent on the ambient current field. Secondly, further Lagrangian simulations with biological processes, forced by different hydrographic conditions in 2003-2005, were conducted. Temporal variations in distributions and sizes calculated by the model corresponded approximately to those of catch data. A large number of particles simulating squid were distributed over the nearshore region from the San-in coast to the Noto Peninsula during SeptemberNovember every year. Differences in the migration route and distribution of particles among years were closely related to differences in hydrographic conditions such as the position of the main stream of the Tsushima Warm Current. Prediction of the catch off the San-in coast might be improved by considering hydrographic conditions in the southwest Sea of Japan.