Recently, domestic and overseas LCC's (Low Cost Carriers) increase air-traffic in Japan. However, we have narrow domain and many airports. Large training areas of USAF or JSDF also exist in the airspace. In the present circumstances, the flight safety is kept by separating airways from training areas. In the near future, increasing number of flights will make severe situation. In this paper, we try to optimize the evasive courses of aircraft in the narrow airspace. We formulate collision avoidance as a nonlinear programming problem and analyzed numerically. The optimal solution we obtained show courses in narrower spaces than ordinary ones. The assumption of limited airspaces is useful in severe situation like bad weather. Thus, this study might be a good proposal to decide direction for collision avoidance.