In recent years, intense rainfall has increasingly caused catastrophic slope failures and debris flows. Numerical modeling of such phenomena requires reliable simulation capable of handling complex multiphase interactions among water, boulders, and vegetation with highly deformable free-surface flows. In conventional fluid–rigid body interaction analyses, hydrodynamic forces are evaluated via surface integration using boundary normal vectors, which becomes problematic for complex or irregular geometries due to discontinuities. To address this issue, this study proposes a Smoothed Particle Hydrodynamics (SPH)-based fluid–rigid body coupling method that combines the SPH(2) formulation [1] with the Fictitious Domain (FD) approach. In this framework, rigid bodies are treated as fictitious fluid regions, allowing hydrodynamic forces to be evaluated by volume integration without explicit interface normal vectors. The method is validated using the classical settling of a circular disk [2]. Although minor discrepancies appear due to particle-based volume representation at coarse resolutions, a volume correction procedure significantly improves agreement with the reference solution. A more practical validation is performed using experimental observations of the settling motion of complex-shaped rigid bodies, specifically a L-shaped object [3]. The proposed method accurately reproduces both translational and rotational motions, demonstrating its robustness and predictive capability. These results confirm that the FD–SPH framework reliably evaluates fluid–structure interaction forces for objects with complex geometries. Furthermore, the approach is applicable to slender and branching structures such as trees, indicating strong potential for simulating debris flows with driftwood in natural hazard contexts.