Large wood (LW) disasters, which often accompany sediment-related disasters, occur worldwide. To prevent and mitigate such disasters, we developed a unique LW trap featuring horizontal rods aligned with the flow. When LW enters the trap, it is scooped up by the rods and thus separated from water and sediment. We explored trapping efficacy using a flume of slope 0.087. Water circulated at a constant rate of 1.8 L/s, LW was added to the flow, and the trapping rates were measured. We focused on the relative wood length (Lw) with respect to the horizontal rod spacing (Sr), the number of LW units supplied, and the supply rate. A longer relative length (Lw/Sr) of LW was associated with a higher trapping rate. The trapping rate was also high when the LW number or supply rate was high. The critical Lw/Sr value was 1.5; below this value, LW was not trapped. This study yields the basic information needed to design traps featuring horizontal rods to mitigate LW-related disasters.