The snowmelt-type volcanic mudflow, in which the heat of a volcanic eruption melts the snow on the slope and flows down as a mudflow, is a dangerous hazard that may cause enormous damage downstream. In this study, to elucidate the generation process of snowmelt-type mudflow, we conducted an experiment considering the mixture of heated gravel and snow. As a result of the experiment, the snow melted rapidly under the condition of mixing the heated gravel and snow. It was also found that the smaller the particle size of the heated gravel, the higher the snowmelt discharge. To support these results, we conducted a thermal conductivity analysis within a gravel particle in a spherical coordinate system and estimated the rate of the snowmelt. As a result of the analysis, it was well estimated the experimental results with the different heated gravel diameters.