This paper presents the unsteady internal flow in a mixed-flow pump with a vaned diffuser. Tested pump has a positive slope of the head-flow performance curve at about 65% flow rate of the best efficiency point due to rotating stall in the vaned diffuser. In order to investigate this phenomenon, pressure fluctuation measurements and dynamic PIV measurements have been performed. It is found that the strong vortex develops at the leading edge of the diffuser vane and backflow from the outlet of the diffuser vane is induced and then the backflow conflicts against the outward flow from the impeller. As the result, the periodical unsteady flow is obtained. Based on PIV pictures, it is clarified that a stall occupies four passages (backflow area is three-passage and the stall core occupies one-passage) out of thirteen passages.