In debris flows, larger particles are characteristically concentrated at the front of the flow. To elucidate the mechanics of this, we carried out flume experiments with sediment mixtures, where the flow length, bed roughness and angle of the flume were varied. The experiments showed that 1) Larger particles concentrated more at the front of debris flows with a longer flow length and rougher bed. And the angle of flumes which larger particles concentrated the most at the front depends on a flow length. 2) In the flow direction, the velocities of smaller particles in the front of debris flows were larger than that of larger particles, but in the depth direction, larger and smaller particles had similar downward movement and smaller particles had more downward movement than larger particles in some cases. 3) During the flow down, the proportions of particles with larger diameters increased in the front of debris flows, whereas the proportions of particles with smaller diameters decreased. Based on these observations, we postulated that the concentration of larger particles at the front of debris flows is affected in the following manner. 1) First, in the front of debris flows, smaller particles drop toward the lower layer through the spaces between the particles in the front, whereas larger particles cannot drop out because their diameters are greater than the spaces. 2) The smaller particles that drop toward the lower layer have slower velocities in the flow direction than the larger particles, and subsequently move behind the front of the debris flows. 3) Larger particles remain at the front of the debris flow, and smaller particles are removed. 4) Because the proportions of the larger particles at the front increase the spaces size between these particles increase. Consequently, it is possible for larger particles to drop toward the lower layer. By looping through steps 1) to 4), larger particles become concentrated at the front of the debris flows. The mechanics of above show that the concentration of larger particles at the front is mainly caused by decreasing of the proportions of smaller particles because of dropping.