In order to develop a simple electrostatic separator for solid mixtures, discharge rates of particles through orifices in each end-wall of a horizontal rotating conical vessel were measured for a binary mixture of polypropylene and polystyrene particles which were loaded in the vessel. Acrylic resin (PMMA), stainless steel and polypropylene were used as the end-wall materials of the conical vessel. The particle discharge rates were strongly dependent on the humidity of surrounding air, the electrification characteristics of particles againt end-wall materials (e. g., polarity of particle charge), and the axial segregation characteristics of particles in the vessel. The binary solid mixture with almost the same density could be separated effectively by using the both effects of the electrification and segregation of each component particles in the mixture during their cascading flow in the rotating conical vessel.