When a volute pump is operated under various off-design conditions, the impeller sufferes radial thrust due to the pressure varying around the impeller. The author analyzes the pressure distribution around the impeller under the assumption that the total pressure and the velocity of the fluid leaving the impeller depends upon the local static pressure. In the analysis the impeller periphery is replaced by a straight line, the volute by a wedge shape space, and the flow in the wedge shape space is disintegrated into potential flows and a flow with vortices so that the combined flow many satisfy the boundary conditions along the impeller periphery as well as along the volute wall.The theory predict that the pressure near the tang is contant regardless of the flow rate of pump and that the pressure variation along the periphery is little near the tang and is significant near the exit of the volute. These predictions agree well with experimental facts quoted in the literature.