After volcanic eruptions, debris flow can be triggered by minimal rainfall, which would other wise never cause a debris flow. This suggests that the infiltration rate of the volcanic ash covering the original soil surface is very low. Low infiltration rates of volcanic ash are anticipated due to the presence of clay. This research investigated the hydrological characteristics of volcanic ash deposits to observe the changes in the saturated hydraulic conductivity and infiltration rate under the influence of the presence of clay. Laboratory experiments were carried out on the three kinds of material: (1) volcanic ash from Mount Unzen, (2) volcanic ash from Mount Sakurajima collected at a nearby riverbed, and (3) a mixture of volcanic ashes from Mt. Sakurajima and Mt. Unzen. The results showed that the infiltration rate of the volcanic ash from Mt Unzen was low; approximately 14 mm/hr. It was approximately one third smaller than the infiltration rate of the volcanic ash from Mt. Sakurajima. Also as the clay content increased, the saturated hydraulic conductivity and constant final infiltration rate decreased. The clay content change of low-clay-content ash had a greater influence on changes in the saturated hydraulic conductivity and constant final infiltration rate than the clay content change of high-clay-content ash.