Fluid forces on a centrifugal impeller rotating and whirling in a volute were analysed. It is assumed that the number of impeller vanes is so large that the flow in the impeller is perfectly guided by the vanes. Calculations are made for various flow rates and whirling velocities and the results are compared with experimental values. This model predicts the unsteady forces fairly well and simulates most of the important effects found in the experiments. It has been made clear that the unsteady volute/impeller interaction enlarges the unsteady forces and brings about a destabilizing effect on whirling motion.