It depends on flow patterns formed in pool-and-weir fishways whether fishes succeed in the access to upstream or not. However we had not focused on the relationship between the flow patterns and the swimming behaviors which is a key element concerning about the fish accessibility. As a result, although the pool-and-weir fishways have been installed in many weirs, we could not have evaluated the efficiency. With these backgrounds, eight two-dimensional flow patterns were formed by changing the pool length and pool depth, and swimming behaviors was observed in each flow patterns in this research. It was confirmed that there are three obvious swimming routes to access a upstream pool, and significantly it was certain that one route is more successful to access than the other routes. The result shows that the flow patterns in the pool could provide a fundamental element concerning about fish accessibility and that we have to design the pool length and depth to keep the successful route.