Due to rainfall during the snowmelt season in May 2012, landslides occurred at Nakayama Pass, which is on the main route between Sapporo and Southern Hokkaido, and all lanes there were temporarily closed for the first time since the route opened in 1969. Quantitative evaluation for the risk of landslide such as that at Nakayama Pass during the snowmelt season has remained an issue. In this study, we proposed a method for quantifying the soil moisture of potential landslide sites by comprehensively estimating the long-term water cycle of the watershed. As the result of applying the proposed method to the Nakayama Pass landslide site, it was clarified that the soil moisture of the landslide site at the time of the May 2012 disaster was the highest in the previous 33 years. This proposal was considered effective for quantitative evaluation of the soil moisture associated with landslide during the snowmelt season.