The Aso volcanic deposits in Inukai area mostly consist of the Aso-4 pyroclastic flow deposit, which is divided into A4 and A4W. A4 overlays A4W and mainly composed of tuffs, whereas A4W, of welded tuffs. Erosional process of volcanic deposits may cause slope failure hazards requiring continuous monitoring in order to manage probable risk over people, surface affairs and infrastructure. Erosional process is studied by utilizing 1:50,000 geological map and, digital elevation model processed in geographical information systems (GIS). The present topographic surface was obtained from all the elevation points, whereas the contact surface between the A4 and A4W rock units was generated by utilizing spline interpolation involving only elevation points along the A4-A4W geological boundary. The present topography and the A4-A4W geological contact surface were performed to calculate the total thickness of A4 unit. Generally, the A4 rock units is characterized as poorly welded and lower resistance to abrasion than A4W, so the distribution would be constrained by regional erosion. GIS analysis has made it possible to evaluate the thickness distribution of the A4 rock unit and erosional process in this area. Comparison between these results and the distribution of slope failure risk zones (which were defined accordingly with the methodology to analyze high angle risk zones from the Japanese Ministry of Construction) would allow reclassifying slope failure risk zones
