When an electronic control system has been used for long time, the electronic control system becomes obsolete and no longer In production. In this case, the existing control system should replace by new system due to shortage of spare modules. However, if the proper number of spare modules are stocked, it may be possible and more economical than to replace by new system to extend the usage period of the existing control system. In this case, it is an important problem to determine the proper number of spare modules. In this paper, a method to determine the proper number of spare modules is presented with an application example.