The slope failure/collapse in mountainous region in Japan is one of the major hazards which cause serious damages to the various important structures/infrastructures such as road and railway and may also result in accidents, fatal or otherwise. Recently, slope failure/collapse due to local torrential rainfall in a short term (so called “guerilla like rainfall”) were reported in Japan. From engineering point of view, it is important to investigate the mechanism of slope failure due to torrential rainfall in a short term (including the rainwater infiltration mechanism and its effect on the mechanical behavior) and to estimate the post failure behavior of slopes (including the moving distance and energy of the collapsed rocks/soils). So far many researchers investigated the slope failure caused by rainfall using numerical method (in most case using continuum based approach such as finite element method (FEM)) (e.g., Cai et al., 1998; Cho and Lee, 2001; Huang and Jia, 2009). However, since discontinuum based approaches can model discontinuous planes explicitly and treat large displacement/ movement of rock/soil masses easily, discontinuum based approaches such as discontinuous deformation analysis (DDA) will be suitable to investigate the post failure behavior of slopes. DDA has been applied not only static but also dynamic analyses, for example, rock falls and slope stability against seismic force (e.g., Sasaki et al., 2005; Shimauchi et al., 2009). However, in the past, few researchers applied DDA for slope collapses caused by rainfall because there are still some difficulties to estimate the influence of rainwater infiltration on the mechanical properties.