The efficiency of granular bed filters and a critical operational condition due to particle entrainment from the filter were studied. The subject of the present study was concerned with the filtration of highly concentrated dust particles, in which the dust entrainment takes place dominantly at the very beginning of filtration. A model has been proposed to describe the filtration process accompanied by dust capture and reentrainment, based on the evaluation of drag force acting on particles in contact with a granule. Since, in the model, the granular bed filter was regarded as consisting of apparent partial filters in series, the local amount of deposited dust and therefore partial efficiency could be predicted. Removal of fly ash was carried out using spherical granules at constant filtration rate. The overall efficiency of the filter was lower at higher gas flow rate, although the filtration proceeded in the range that the effect of inertial impaction is dominant. The decrease of the efficiency has been related to the degree of particle reentrainment. The model was verified with the experimental results obtained under various conditions of gas velocity, granule diameter and filter thickness, and was effective to explain the decrease of collection efficiency with dust load.