This paper describes a self-excited vibration of a Flow Dynamic Conveyor (FDC). The FDC is often used in power plants and in iron works because of superiority in noise control compared with a roller type conveyor. The FDC consists of a trough and a belt, and the air is supplied from many holes provided on the trough. A large vibration suddenly occurs when the flow rate becomes the critical value. In this paper, some measurements were performed in order to clarify the phenomenon. As a result, it was found that the phenomenon was the self-excited vibration based on the interference between two vibration systems such as the load·air-spring system and the trough·supporting system.