We propose an efficient network design algorithm that realizes shared protection. The algorithm iteratively improves the degree of wavelength resource usage and fiber utilization. To achieve this, we newly define two metrics to evaluate the degree of wavelength resource usage of a pair of working/backup paths and the fiber utilization efficiency. The proposed method iteratively redesigns groups of paths that are selected in the order determined by the metrics. A numerical analysis verifies that the proposed algorithm can substantially reduce the required wavelength resources and hence fiber cost. It is also verified that the computational complexity of the proposed algorithm is small enough to terminate within practicable time.