A parallel detailed router based on the area division is one of important tools to overcome the increase of CPU time required for routing of a very large multilayer SOG. In order to conduct routing in each divided area independently, fictitious terminals are introduced on the border of each divided area, and routes connected to the fictitious terminals are concatenated to complete the final detailed routes. In this paper, we consider a problem how to position such fictitious terminals on borders, so as to make each detailed routing in a divided area easy. We formulate this problem as a minimum cost assignment problem, and propose an iterative improvement algorithm. We also give some experimental results which indicate the effectiveness of the algorithm.