When extra-large earthquake which exceeds the input level of designing occurs, a base-isolated building may collide with surrounding retaining wall. To evaluate accurately the acceleration of superstructure and the deformation of base isolator at that time, restoring force characteristics of the retaining wall including the back soil should be clarified. In this study, the collision analyses of base-isolated building using 3D-FEM are carried out, and the restoring force characteristics of retaining wall are investigated. Simple modeling of the collision spring which have the initial rigidity and the maximum strength are proposed as for parameters of height and length of retaining wall, collision speed of building, collision height, and the shear wave velocity of back ground.<br> The obtained results are summarized as follows.<br> 1) By collision analysis and evaluation of restoring force characteristics of a real base-isolated building model, retaining wall thickness, height and length greatly affect retaining wall restoring force characteristics. Initial stiffness and maximum load stress of retaining wall portion increase when the retaining wall thickness, height and length are increased. On the other hand, the weight of building and the ground at the bottom of the foundation board have little influence on the restoring force characteristic of retaining wall. In addition, the collision speed of the building exerts a large influence on the restoring force characteristic, and the initial stiffness increases as the collision speed increases, but when it reaches a certain collision speed or more, it reaches a certain limit and becomes constant. The maximum load increases with the square root of collision speed. Since the collision speed varies with the assumed input seismic motion, it is important to perform the earthquake response analysis on the building in advance and to use the speed at the time of displacement by the clearance between the building and the retaining wall. The initial stiffness and Maximum load increases as shear wave velocity increases.<br> 2) We propose Equation 7 for initial rigidity and Equation 8 for maximum load as a simplified retaining wall collision evaluation formula. By this formula, it is possible to install the collision phenomenon of the retaining wall of the base-isolated building in parallel with the spring of the base isolation device as a collision spring, by changing the shape (thickness, height, length) of the retaining wall and collision condition (Height, impact velocity), and the surrounding ground (shear wave velocity).