Snowmelt in early spring is one cause of slope failures and landslides in snowy regions. The infiltration of melt water into a cut slope along an expressway plays a critical role in reducing resistance to sliding. Despite its significant impact on snowmelt-induced slope failure, snowmelt runoff process has not been discussed in most specifications. Through simple laboratory testing on snowmelt and observation of snowmelt processes in the natural topography, this paper examines the case of a recent cut slope failure in the Hokkaido Expressway in the snowmelt season, and describes how snow melts and how melt water infiltrates the ground to destabilize the cut slope. Snowmelt runoff tests were conducted both inside and outside the laboratory, focusing on the fact that when a small snow block melts, melt water drips only from the lowest part of the block. As part of the laboratory tests, snow blocks were placed on two slopes made of sandy soil. While one slope was completely covered with snow blocks, the other slope was covered only the upper part of it. As the result of the tests in the former case, melt water moved down slope and flowed out from the snow layer, leaving the soil slope in a sound condition. In the latter case, melt water flowing out of the lowest part of the snow layer infiltrated into the sandy soil, resulting in a failure. Snowmelt processes similar to those observed in the laboratory tests can be seen on natural slopes. Since the site of the expressway cut-slope failure mentioned above is located at the lower end of a snow-covered area of an expressway cut slope, it can be inferred that phenomena similar to those observed in the laboratory tests occurred at the slope failure site.