The maintenance cost should be considered in the early planning phase of process design for reducing it, and a cost estimation method is essential to evaluate alternative design. The shutdown maintenance of condition basis maintainable equipment modules is concerned here, then that maintenance plan through the plant life should be optimized on the basis of expected degradation curves of these equipment modules for an accurate process evaluation. The shutdown timing and grouping of equipment modules, which are maintained at each shutdown period, become decision variables, and these variables should be optimized simultaneously under the constraint of the degradation curves. In this study, the relation between these variables through the degradation curves is considered, and a sub-model for sequential two shutdown periods for any equipment module is provided by introducing 0/1 integer variables, and it is expanded for overall equipment modules and whole plant life. This model is linearized from the consideration of required accuracy in process evaluation, and an integer programming model for the shutdown maintenance planning is developed.