When conduits under roads were blocked by cobbles discharged from mountain area during heavy rainfall, flood started to flow on the roads and damage them such as the disasters in Fukuoka and Oita Prefectures in 2017. In such situations, it would be very difficult for residents to evacuate to their shelters safely. However, the process and mechanism of sedimentation in a conduit is still unclear, so that we aim to make clear the process and mechanism of sedimentation in a conduit by flume experiments and three‐dimensional numerical simulations. We used beads instead of cobbles in our experiments. We found followings; (1) the beads suddenly became slow after passing the hydraulic jump, (2) the following beads collided the slowed beads and they started to deposit, and (3) the direction of forces acting on beads such as pressure and drag force was to upstream, so that the beads became slow and stopped.