This paper explains how the debris originating from the sector collapse of Asama volcano at 24ka, traveled longer than 90km as a kind of “plug flow” with a thin laminar boundary layer. At 3 outcrops at distances of 45km, 75km, and 90km from the origin, vertical changes in the chemical composition throughout the debris avalanche deposits and underlying fluvial sediments were examined employing the energy-dispersive X-ray spectroscopic method. 57 samples were classified into two groups with different chemical compositions. The chemical composition of the debris avalanche block with fragile internal structures at the 90km site, is almost the same as those of the debris avalanche matrix. This indicates that the debris avalanche matrix originating from the collapsed volcanic body was partly produced from the debris avalanche blocks and that almost all portions of the debris avalanche deposits consist of the plug part. The lowermost part of the debris avalanche deposits at the 45km site, however, expresses a chemical composition similar to that of the fluvial sediments. This indicates that the debris avalanche, having a laminar boundary layer with strong shear stress working at the base, was flowing down. The geochemical result and lithofacies of the deposits suggest that the sediments have traveled rapidly at least 90km, as a single debris avalanche.