This paper presents a method for planning motions of a flexible objects based on precise simulation using Finite Element Method (FEM). The proposed method is applied to ring-shape objects manipulated by robot arms, which is often seen in various applications. Since large deformation is implied, assembly planning with realistic simulation is important to ensure task efficiency for the robot and also to avoid damage of the object. We first verify that the behavior of a ring-shape object by dual-arm manipulation is well predicted using FEM model of bent beam through a simulation along the trajectory computed by optimization-based motion planning previously reported. Next, a precise FEM model is integrated into optimization to compute a trajectory of robot hands minimizing the deformation energy as well as satisfying such criteria as collision avoidance and smoothness. Since the direct computation leads huge computational cost, we present a realistic formula which transforms the planning problem into the static equilibrium problem of several FEM models located along the trajectory. Simulation results show that the proposed method is promising for such assembly tasks requiring large deformation.