This study estimates the collapse mechanism of an experiment that reproduces the destruction case of a steel open sabo dam using distinct element method (so called, DEM). In the experiment, a steel pipe open sabo dam was destroyed by a debris flow in Nagiso Town, and an steel pipe open sabo dam modeled in concerning with compressed deformation occurred is made of Barsa material. As a result, the compressed deformation occurred and the dam is damaged when the height of the preceding sedimentary gravel is 30 %. DEM analysis revealed that the axial force of the upper horizontal member increased in case of 30 % height of the preceding sedimentary gravels. Then the upper support diagonal member were broken due to bending force.
