In a fluidized-bed operation, unwanted particles larger than a designed size can be introduced unexpectedly or form during the operation in a reactor. These large particles should be removed from the bed during the operation, otherwise a long-term stable operation is impossible due to defluidization phenomena caused by them. For this purpose, the present study examined a spout-fluid type fluidized bed reactor of which conical gas distributor had a spout tube(outlet) in its center, using glass beads as solid particles to be simulated. In the removal of large particles from a fluidized bed, the gas velocity in the spout tube was found to be one of the most important factors. In order to obtain more than 90% separation efficiency of unwanted coarse particles from fine particles, the gas velocity in the spout tube had to be kept at least 3.0 times higher than the terminal velocity for the largest particle size of fine particles. The separation efficiency decreased as the portion and the size of coarse particles decreased and as the fluidization in the bed became worse. It was also found that if the discharge of coarse particles through the spout tube was not affected by the decreation of inner diameter of the tube, the smaller inner diameter of the tube showed the better separation efficiency at a low gas velocity in the tube.