Increasing frequency of sediment―laden flood, risk assessment and countermeasure plans based on riverbed variation calculations are important to reduce hazards. To improve the accuracy of the calculations, it is important to predict the deposition process of debris flows. The authors conducted experiments using an experimental flume with a gradient change point to observe riverbed variation changing experimental conditions. A high‐speed camera was installed to measure the deposition rate at the gradient change point of the flume. In addition, water and sediment in the flume were directly collected to measure the transport concentration of sediment. The relationship between particle size and deposition rate from experiments is shown in this paper. Collected data are also compared with the deposition rate from conventional formulae. Positive correlation between the deposition rate from the formulae and the experiments, and range of the deposition rate coefficient in the formulae are shown in this paper.