A thickener-modeled dry dense medium separator using a gas-solid fluidized bed was developed in which materials of different specific gravity float or sink in the bed and can be withdrawn by overflow or underflow. This system is expected to allow high-speed separation and low-maintenance operation compared with a conventional dry dense medium separator, because there are no moving parts for withdrawing floats and sinks from the bed. The effects oh overflow/underflow rate and sample feed rate on the performance of the separator were investigated using silica sand as a fluidized bed medium and 20-mm diameter spheres of various specific gravities as samples for separation. It was found that when the overflow/underflow rate was 0.44 or less, the float-sink boundary was smaller than when there was no overflow and no underflow but float-sink sharpness was maintained. On the other hand, when the overflow/underflow rate was 0.64 or more, the float-sink boundary was similar to that in the case of no flow, but separation sharpness deteriorated. Furthermore, when the overtlow/undertlow rate was 0.44 and the feeding rate was 0.035 kgis (=2.2 m(3)-object/(m(2).h): volume-based separation speed per fluidized bed area) or less, Newton efficiencies were more than 0.9. Compared with conventional apparatus, the separation speed of the present apparatus was 3.4 times higher. Hence, this withdrawing system can be valuable for increasing separation speed and reducing the need for maintenance.