The purpose of this study is to accumulate data to predict the ruptures of aneurysms on the bifurcation of the middle cerebral arteries at the base of the brain. Particular stress is laid on understanding the elemental nature of branch flows with/without an aneurysm. Therefore, “flow patterns” and “wall shear stress”, which are important factors for the causes of ruptures, are investigated by the three-dimensional experiments in vitro and the two-dimensional numerical simulations with simplified models. In the branch arteries without an aneurysm, there is a possibility of growing aneurysms at the location slightly away from a stagnation point. If an aneurysm forms into a centrosymmetric shape for the inlet axis, it is considered that they tend to grow further in a symmetrical plane. From the viewpoint of the risk of ruptures, recirculation flows become problematic with the lower Reynolds number, while the influence of wall shear stress becomes larger with the higher Reynolds number.