PURPOSE: To prevent a clearance from being blocked by foreign matters easily by arranging a clearance closing slidable member and plural engagingly locking members engaged each other to be used when the clearance becomes zero, in the clearance between an impeller vane and an impeller hub.

CONSTITUTION: An operation rod 7 is arranged in a pump main shaft 8 whose upper end is fixed to a blade angle operation mechanism so as to be driven. An impeller vane 1 is installed to the lower end of the pump main shaft 8 and rotated through the operation rod 7 and its blade installation angle is adjusted. In the above composition, the impeller stem shaft 20 of the impeller vane 1 is supported by each of upper and lower bearings 3a, 3b. And a clearance closing slidable member 9 is arranged in the clearance between the impeller vane and the impeller hub 2. Engagingly locked members 13a, 13b are formed on the outer circumference of the impeller stem shaft 20 and the inner circumference of the lower bearing 3b respectively and they are engaged each other when the clearance 12 becomes zero.