Two types of tunnel model tests, failure test and constant load creep test, were executed on a manmade soft rock model ground made of a mixture of gypsum and diatom whose mechanical behaviors are very similar to soft sedimentary rock. Meanwhile, finite element analyses based on an elasto-viscoplastic model, which can properly consider the influence of the intermediate principal stress, are conducted to simulate the model tests. The simulated results indicate that the finite element analysis based on a suitable constitutive law can appropriately describe the long-term deformation behavior of tunnels in soft rock.