In this study, we focused on the phenomenon that stream bed sediment turns into debris flows and clarified under what circumstances the risk of the occurrence of such debris flows increases. We proposed and adopted a method for evaluating the occurrence of debris flows even in situations where surface flow exists over unsaturated sediment, which had not been done in previous studies. Improvements were made to Storm Induced Multi‐Hazard Information Simulator (SiMHiS), the sediment dynamics analysis model so that the water depth of the surface flow and that of the subsurface flow in channels were obtained by analytical calculation. The target watershed was the Hiru‐Dani, which belongs to the Jinzu River system and is located in Takayama City, Gifu Prefecture, and was analyzed in 300 different cases with various values in five parameters in order to study the effect of each parameter. As a result, it was found that Manning’s coefficient of roughness and infiltration capacity in the channels do not significantly affect the occurrence of debris flows, and debris flows are likely to occur when the sediment layer thickness is large, the hydraulic conductivity is small, and the cohesive force is small.