The distinct element method (DEM) is a simple numerical model which is capable of describing the motion of granular materials. In this study, the motion of particles discharging from a rectangular hopper has been numerically simulated by the DEM whose time steps are Delta t = 10(-6), 10(-5) and 10(-4) s, which correspond to stiffness k(n) = 7.0 x 10(7), 7.0 x 10(5) and 7.0 x 10(3) N/m,respectively. The calculated results have been compared with the experimental data. All of the calculated results not only qualitatively describe the experimental results, but also approximately describe the quantitative characteristics of the measured data, although the stiffness of the calculations is greatly different from the real stiffness of the materials. However, the calculated result of the DEM using the smallest time step (Delta t = 10(-6) s) could not give the best description of the experimental data. This suggests that the calculated results of the DEM could not accurately describe the real granular how even if the time step is small enough to use the real stiffness of the materials. The most significant approximation of the DEM is that distinct particles do not interact with any other particles and displace independently from one another during one calculation time step. The discrepancy between the model based on this approximation and the real granular flow would not decrease as the time step is decreased.