|The present study proposes a method to evaluate a transport process of driftwood in mountainous rivers. The erosion and deposition rates of driftwood are formulated using sediment erosion/deposition rate formula for debris flow. We describe the transport process of driftwood by means of a convection equation with the formulated erosion and deposition formulas. The transport process of driftwood is solved numerically by integrating the convection equation and governing equations of debris flow simultaneously. The numerical result shows a good agreement with data observed in the 2017 rainfall event of Northern Kyusyu at the small tributary of Akatani River.