Fallen objects on highways are one of the direct or indirect factor which induce major accidents. Therefore, in management of highways, the improvement in visibility to fallen objects has been an important issue. And, the reflectance distribution of fallen objects and the tunnel lighting system greatly influence the improvement in the visibility of fallen objects. In this study, it is set the eventual goal that the establishment of the decision making of the tunnel lighting system, and as its basic study, it is precisely estimated the reflectance distribution of fallen objects. Specifically, it is represented the reflectance distribution by beta mixture model that can be considered multimodal. Mixing coefficient and unknown parameters of beta mixture model are estimated by the EM algorithm. Finally it is indicated the application example which used the survey datas, and verified of the validity of the model in this study.