In this study, the internal flow field in a mixed-flow pump at the flow rate where performance instability occurs was investigated using CFD, and the internal flow field in the impeller and the vaned diffuser of the pump was visualized with a specific speed of approximately 1,000 (min⁻¹, m³/min, m) by PIV measurement. The flow pattern in the vaned diffuser did not differ markedly compared with the flow rate was slightly large just before the head drop in experimental results and computational ones around the performance instability. However, the flow pattern of the impeller differed. In the case of instability occurrence, a recirculation flow that reached the trailing edge of the impeller blades was observed. From the results of experiments and computations, it was confirmed that the drop of the pump total head at the point of performance instability was mainly caused by the difference of the flow pattern in the impeller at the flow rate before and after the head drop.