In the maintenance planning, the maintenance cost is reduced by working for multiple bridges simultaneously. However, it is difficult to establish an optimal plan that reduces the maintenance cost by this work because there are a large number of bridges that requires the management. In this study, an attempt is made to propose the planning method for a large number of bridges in two stages by using Genetic Algorithm. In the first stage, the proposed method optimizes a maintenance plan for single bridge with considering the preventive maintenance. Here, this optimal plan involves the period in which the work can be postponed without losing the safety. Therefore, in the second stage, the proposed method minimizes the maintenance cost by considering the year of works in the postponable period of multiple bridges.