The FDC (Flow Dynamics Conveyer) has often been used in power plants and iron works because of its superiority in low noise and power compared with roller type conveyers. The FDC consists of a trough and a belt, and the air is supplied from a series of holes provided on the trough. However, large vibrations occur when the flow rate reaches a certain value. This abnormal vibration is defined to be self-excited vibration caused by leakage flow. In this study, as a first step, the theoretical model of a non-tapered clearance flow passage is examined and the natural frequency is calculated. In addition, experiments are conducted to examine the validity of the theory. As a second step, in the case of a tapered clearance flow passage, a concentrated load is added to the belt. The effects of the belt deformation, the flow rate, and the added loads on the clearance flow are examined along with considering the generation mechanism of the abnormal vibration. As a result, it was clarified that the generation mechanism of the self-extracted vibration was due to the phase shift by a change in parameters.