We investigated how hardness of the material inside a balloon used for endovascular occlusion to treat cerebral aneurysm. This study was motivated by the fact that the balloon occlusion employing HEMA (2-hydroxyl ethylmethacrylate) can lead to further growth of aneurysm in many cases. We showed distribution of stresses acting on the cerebral aneurysm wall by means of a dynamic finite element method (FEM). We used a 2-dimensional finite element model assuming the conventional type of cerebral aneurysm. Our simulation results indicated that the optimal Young’s modulus of the material inside the balloon is about 100 kPa, which is much lower than that of HEMA. We believe that our simulation method can be used for a specific patient condition and provide a proper value of the Young’s modulus of the inside material.