In Kanayama-sawa, which is located in the Hime-kawa basin, unstable soil masses, with an estimated soil volume of 720,000 m³, and movement of soil were observed via on-site inspection and from data acquired from the laser profilers(LPs). The unstable soil masses might cause damage downstream, since they are prone to fluidization due to snowmelt runoff or heavy rain. Therefore, in this study, we observed the soil movement in the field and employed debris flow numerical simulation to determine the effect of debris flow caused by fluidized soil masses. From the on-site observation, we found that the unstable soil-mass moves 1.6 m per month at maximum, and that the soil mass can be divided into four blocks. In simulations, we applied three scenarios of debris flow occurring from this soil mass by varying the soil volume-small, medium, and large. The debris flow outreach varied with soil volume, and was thus different in each case ; however, flooding was limited to the current river channel in all cases.