A futuristic air transportation system should cope with high economic efficiency while ensuring safety of its users and customers. Hence, conflict resolution between airborne aircraft is necessary to achieve fuel optimized flight trajectories. In this study, as an approach for conflict resolution, we calculate the separation between aircraft and evaluate a degree of conflict by acquiring Secondary Surveillance Radar Data measured for 24 hours in the Kanto region. Moreover, we devise an algorithm for the purpose of revealing how air traffic controllers avoid aircraft interferences. In this method, we estimate control procedures by air traffic controllers and review results as the preliminary stage towards the development of a conflict-free trajectory optimization algorithm.