PURPOSE: To enable easily increasing the inertia moment of a rotation body and eliminate unstable work during assembling.

CONSTITUTION: Main impeler part 6 is provided at the upper part and an in- water motor 31 driving the main impeler part 6 is provided at the lower part, these rotation axis are assembled in one, and a fly wheel 32 is placed detachably and rotatably in one putting a coupling bolt 33 in between at the lower end of the motor axis 13 of the in-water motor part 31. This fly wheel 32 has a reverse U shape in the vertical cross section and a dimension of diameter and axial length capable of obtaining a desired inertia moment, and is provided with a plurality of pump holes along the radial direction.