The main portion in the deposits of pyroclastic flow shows various characteristic features such as density and size segregation of large fragments in the vertical direction. To understand the mechanism which produces these features, we numerically study granular flows on an inclined rough surface with impurities that are different in density and size from the matrix particles. The simulation shows the occurrence of density and size segregation of large impurities, and the impurities finally concentrate at the top and the bottom. Based on these results, we discuss the mechanism of the segregation. It is found that the behavior of impurities is governed by gravity, buoyancy and the viscosity of the randomly moving matrix.