The unsteady behavior of a vortex generated on the diffuser leading-edge, which is called the leading-edge vortex (LEV), is discussed through experiments and numerical simulation. The LEV is different from the separating vortex of the diffuser leading-edge and passage vortex of the diffuser, develops rapidly with a decrease in the compressor mass flow rate, and forms a flow blockage in diffuser passages. Therefore, the evolution of the LEV may become a cause of diffuser stall. Additionally, in one attempt to control the LEV, two types of tapered diffuser vanes, which are shroud- or hub-side tapered diffuser vanes, were adopted. Though the shroud-side tapered diffuser vane can effectively reduce the compressor noise level, the compressor performance deteriorates remarkably. On the other hand, the hub-side tapered diffuser vane not only reduces the compressor noise level but also improves the compressor performance. According to the visualization results of the oil-film methods and numerical simulations, the hub-side tapered diffuser vane can suppress the evolution of the LEV in the compressor low-flow-rate operation.