In this paper we propose a mathematical model and its algorithm to build the optimum road sign system that keeps drivers on their original course. We assumed that signs were posted only at the entrance of intersections. Furthermore we defined the Straying Index as an expression of drivers straying and proposed a method for minimizing the total Straying Index of all OD. In this method, the total Straying Index of drivers was taken as the objective function, while the number of links where signs were to be posted and the numbers of displays for one direction on the signboard were treated as the restrictions. We solved this problem through the analysis of a small network example using dynamic programming and extracted some properties of sign type by taking into account such characteristics as place name, route number, and the combinations of both.