New air traffic management systems are being researched throughout the world to enhance the safety and efficiency of the airspace. However, human air traffic controllers have been effectively balancing both safety and efficiency in their operations. In resilience engineering, this human trade-off characteristic is called the efficiency-thoroughness trade-off if efficiency is prioritized and thoroughness-efficiency trade-off if thoroughness or safety is prioritized. Moreover, understanding human decision-making is attracting attention in the field of air traffic control system design since it could be incorporated into autonomous systems to improve resilience and automation acceptance. In this study, we analyzed the mechanism of efficiency-thoroughness trade-off decisions in an approach control simulator experiment, so that the results could be applied to future air traffic management systems. The results showed that efficiency-thoroughness trade-off decisions were made when situational difficulty was under 3.5 times the difficulty of an easiest situation and when situational difficulty was over 3.5, a high ratio of thoroughness-efficiency trade-off decisions was observed.