Construction cost of tunnel is decided on construction area, site conditions, and construction methods. However, there is few research on developing models to estimate early construction costs. Therefore, this paper presents a model for early estimation tool for NATM tunnel using system thinking. The effect factors for tunnel construction cost were defined and the causal map is developed. Then empirical case analysis were performed to identify the cost difference due to tunnel length, excavation volume, and rock quality. The proposed model would be an alternative to estimate early construction cost of NATM tunnel.