When a tunnel is assumed to receive swelling pressure from rocks, it is necessary to estimate the swelling pressure properly and to set the external force condition for the tunnel’s repair design. For the mountain tunnel under service in Fukushima prefecture, there was a deformation at the roadbed and the tunnel lining due to swelling rock. Therefore we developed a swelling-rock model based on onsite monitoring data and laboratory test data. The model was used to estimate the external forces for design. Several monitoring devices were installed in the site to obtain rock deformation data around the tunnel and evaluate the mechanism of swelling. In addition, swelling pressure tests using site materials were conducted to grasp the swelling characteristics of the rock. A numerical simulation model was made for the interaction between the swelling pressure of the rock and the tunnel structure using the rock deformation with the results of the tests The simulation model, which was made in consideration of past data, estimates the external forces, which are necessary for the reconstruction of tunnels in the future. This paper outlines the method of setting the swelling model by numerical analysis, the features of the model, and the results of applying it to a tunnel.