PURPOSE: To suppress the generation of the collision loss caused by the protrusion of guide vanes on the flowing water surface and reduce the frictional loss caused by a slight flow velocity reduction when the machining and assembling errors are within allowable ranges by setting the inlet height of the guide vanes higher than the outlet height of vanes of an impeller at the outlet section of the impeller of a pump.

CONSTITUTION: The inlet height H2i of guide vanes 2 is set higher than the outlet height H1e of vanes 1a of an impeller 1 at the outlet section of the impeller 1, i.e., the relation between H1e and H2i is set to (H2i): 1.1×(H1e). A fluid flowing from the vanes 1a of the impeller 1 can smoothly flow into the guide vanes 2 regardless of some machining error and assembling error. Since the passage area is widened by the guide vanes 2, the average flow velocity is reduced, and the frictional loss is reduced as the secondary effect.