The volumetric expansion ratio of rock mass can explain why the depth of the surface subsidence occurrence is smaller than the height of an existing opening; it is because the empty space of the gangway can be filled with the broken rock. Considering the recent research results, however, the volumetric expansion ratio of rock mass has been calculated mostly by the theoretical equation through numerical models and has hardly been used in the studies of the subsidence prediction and the restoring plan. When the surface subsidence mechanism is studied without consideration of the volumetric expansion ratio, it is usually overlooked that the amount of subsidence occurrence can be overestimated. In this study, the authors researched the subsidence occurrence mechanism with a new theoretical approaching method. The volumetric expansion ratio obtained from this method has been applied to the numerical simulations. Also, numerical analysis was conducted by discontinuum modeling using UDEC(Universal Distinct Element Code) in order to analyze the volumetric expansion ratio and subsidence aspects according to the distribution pattern of the surface of discontinuity.