The Shirate Bridge, an old rock arch bridge located in Oita, has become weathered over the years and is slightly damaged at its base of abutment. This paper describes two-dimensional static and dynamic discontinuous numerical model studies of rock bridge using discontinuous deformation analysis. The discontinuous numerical model of a bridge consists of an assemblage of polygonal stone blocks, abutment, roadbed and rockmass. The logistical function-shaped subsidence curves and then sinusoidal earthquake waves were inputted into the model. Block activities in a narrow area of rock bridge were evaluated with such parameters as: increase in void volume (dilation); horizontal, vertical and rotational displacements; and kinetic energies of rock blocks at different points. The relationship of base subsidence on the failure mode and the development process are presented in detail. Results indicate that the larger the base subsidence, the more discontinuous motions become dominant, and that higher frequencies engender the dominance of vertical and rotational motions of blocks.