A debris-flow brake is an erosion control facility designed to stop debris flows. However, the mechanism of the debris-flow brake has not been explained. This study discusses the change in pore-water pressure near the deck of a debris flow brake which controls solid friction between the debris flow and the deck. A dynamic model that describes the behavior of debris flows at the debris-flow brake was built It incorporates not only the change in sediment concentration of the debris flow due to water draining through the debris-flow brake but also the resulting change in pore-water pressure. Numerical simulations and experiments examined the behavior of debris flows at a debris-flow brake. The simulation and experimental results agreed. The simulation results show that the change in pore-water pressure plays an important role in how a debris-flow brake stops debris flows.