To prevent the occurrence of separation by using an existing impeller in place of designing and manufacturing a new one, avoiding the occurrence of disturbance in flow, an increase in loss in a suction port, and overloading and vibration caused by collision of flows, and thereby prevent the impeller from being damaged.

A plurality of flow rectifying plates 12, 12,... are provided on the inner surface of a suction port 2 at the upstream side of an impeller 4 in a vertical shaft pump 1 to which prewhirling flows are introduced. Each flow rectifying plate 12, 12,... is equipped with an inflow end part faced to prewhirling flows along the whirling direction of a prewhirling flow, and with an outflow end part faced to the entrance 4A of the impeller 4 along the axial line C direction of the impeller 4. And a direction conversion part in a circular arc shape is also provided, which converts in a prewhirling flow direction introduced from the inflow end part gradually to the axial flow along the axial line C direction of the impeller 4.