PURPOSE: To reduce a loss caused by expanding a guide vane outlet for improving pumping efficiency by providing an opening portion in a shroud of the guide vane to bypass flow for forming a jet flow along the outside wall of the shroud.

CONSTITUTION: A shroud 17 of a guide vane 6 is provided with an opening portion 15 and in the downstream side with a shroud 14. Flow introduced from an impeller into the guide vane 6 is divided into one 19 passing through the opening portion 15 and the other 20 passing through the inside diameter side path 17. The flow 19 forms a jet flow 21 along the shroud 14. The flow passing through a main path 17 of the guide vane is reduced due to the bypass flow 19 and reduces speed in proportion to the flow 19 at the guide vane outlet. While the flow speed is reduced at the guide vane outlet, fluid around the guide vane outlet is also accelerated by the jet flow 21 so that the generation of violent shearing flow is alleviated and a loss caused by expansion of the guide vane outlet is reduced.