PURPOSE: To have high efficiency and high suction performance even though the speed in preswirling has changed with changing rate of flow by making movable the tongue part provided at the tail of the volute, by making adjustable the preswirling of the suction stream.

CONSTITUTION: A tongue part 6 is provided at the tail part of a suction volute 2 with possibility of movement. This tongue part 6 is fastened to a rotary shaft 7 provided rotatably at the suction casing 1 in parallel with the axis of impeller 4. The means 8 to rotate this rotary shaft 7 is equipped with a drive device 9 for ex. a motor and arranged to drive the rotary shaft 7 through a power transmission device 10. The drive device 9 is controlled by an automatic controller 12, which receives signals form such an input signal sensor as a flow-meter 11 and a pressure meter 13 installed at the suction pipe 14 and discharge pipe 15, respectively, to turn the tongue part 6 till a certain specified positon for the purpose of making adjustable the preswirling of the suction stream. Thus a pump can be obtained which presents high efficiency and high performance even though the speed in preswirling has changed because of change in the rate of flow.