Collapse of adobe buildings during an earthquakes with various earthquake intensities is analyzed using the 3-dimensional distinct element method code developed by the first author. Firstly, we evaluate structural damage with damage index, interior-space damage with w score, and human casualties in the collapsing buildings with three casualty criteria. Secondly, we investigate relationships between the earthquake intensity, the structural damage, the interior-space damage, and the casualties. It is found that the casualties have stronger correlation with the interior-space damage than the structural damage, and three vulnerability functions, namely, structural, interior-space and casualty vulnerability functions, are proposed. Effects of reinforcing methods on these functions are also examined. Finally, a series of analytical procedures to develop vulnerability functions and to estimate casualties using the functions are described.