Laboratory flume experiments showed the different behavior of sediment grains and wood pieces in debris flows (wood-sediment-water mixture flows) at an open check dam model. In the experiments, the debris flows were caused by putting some wood pieces on the initial movable bed and dropping the other wood pieces on the surface of the subsequent flow part. This condition resulted in the concentration of the former pieces at the flow front and the latter pieces on the subsequent flow surface. The latter pieces trapped in the check dam model were piled up on the former pieces trapped in the check dam. The sediment followed the pieces accumulating at the flow front. At the same time, sediment deposition was caused behind the trapped wood pieces.