In the formation process of debris fan, the interaction between sediment particles is one of the most important factors. In this study, the formation process of debris fan is simulated by using 3D movable bed simulator which is able to express the interaction between particles with tracking indivisual particles. The mechanism of the formation process of the debris fan due to the sudden decrease of bottom slope and the expansion of the width of channel is investigated through the simulation results. The deposition process found in the simulation data reproduces well the characteristics of previous experimental data.