PURPOSE: To enable a device to adjust rolling in right and left directions, by providing a gliding plate, tiling diagonally downward in the flow direction of water and sinking in the water, as a buoyancy mechanism in the supporting part of water turbines while forming said gliding plate offsettable in the rotary shaft direction of the water turbines.

CONSTITUTION: A tightening band 18 is loosened, and if a device adjusts the rotary position of an inner pipe 16 for an outer pipe 17, a gliding plate 10 is turned in the direction of an arrow head B enabling a tilt angle to be adjusted for a water surface W. While the device, if it adjusts the position in the axial direction of the inner pipe 16 for the outer pipe 17, enables the gliding plate 10 to be offset in the direction of an arrow head C in the drawing. The offset adjustment of said gliding plate 10 can suppress rolling in right and left directions of water turbines 6 and a waterfoil 9 generated on the basis of a change of antitorque of a generator 5.