We propose a simple approach for treating non-planar free-surface boundary conditions that can be easily implemented into an existing finite-difference code. The approach is based on the three-dimensional staggered-grid finite-difference method (FDM, hereafter). The proposed scheme gives very good results when compared with other numerical methods. Its accuracy depends on how well the topography is matched by the 3D-FD grid. Based on the proposed technique, we made a qualitative evaluation of the local topography effect on ground motion at KOB-JMA site, Kobe, Japan. The local topography at JMA station amplifies the horizontal ground motion in the frequency range 1--3 Hz by a factor of less than two, and deamplifies it at frequencies around 5 Hz by a factor of five. ¿ American Geophysical Union 1996