Rotating instability in various types of fans, compressors, and pumps is considered as one of the symptoms of unsteady phenomena such as rotating stall or surge, and it is observed before a rotating stall as an amplitude increase in the power spectra of velocity fluctuation and/or radiated noise. In this paper, the cause of rotating instability in a centrifugal blower with a shrouded impeller is investigated through both experiments and numerical simulations. Experimental results show that the rotating instability may be attributed to unsteady vortices rotating along the impeller periphery and that a discrete noise component induced by the rotating instability is mainly caused by the interaction between unsteady vortices and the impeller discharge flow. A significant amplitude increase within a frequency band at almost half the blade passing frequency is found to be caused by an irregular change in the vortex rotating speed as well as by an irregular time interval in the train of generated vortices. In the numerical study, the structure and circumferential characteristics of the rotating vortices are investigated by a visualization technique using Qdefinition. Circumferential characteristics of the rotating vortices may be largely influenced by the steady characteristics of the impeller discharge flow field, which are determined by the geometric configuration of the impeller and the scroll casing.