The purpose of our study is to facilitate the effective and feasible design planning of protection structures against rockfall events. We propose a novel method for placement optimization of rockfall protection structures along a traffic road. A series of three-dimensional rockfall trajectory simulations are carried out and then the results are utilized to calculate the risk value as a product of the rockfall hazard, exposure and vulnerability of structures. The proposed method is capable of determining an optimum placement of protection structures that minimizes the total risk on the entire road, while satisfying a given budget constraint. We apply the method to a case study of an actual site to examine the results of optimized protection structures and discuss the capability of the method for risk reduction.