same time as the torsion spring 5 deforms the rocking shafts 33 of all of the vane bodies 3 rock simultaneously under the influence of the water pressure force corresponding to the engine rotation speed more specifically in the low engine rotation speed region the water pressure force is small and therefore the tilt angle increases in accordance with the elastic force of the torsion spring 5 as indicated by an operation diagram in the high engine rotation speed region on the other hand the water pressure force is large and therefore the water pressure force presses a rotary surface of the vane body 3 against the elastic force of the torsion spring 5 eventually overcoming the torsion spring 5 such that the plate cam rotates to an opposite side in the rotation direction as a result the tilt angle of the vane body 3 decreases such that adjacent vane bodies 3 in the rotation direction approach each other closely as described above the torsion spring 5 is housed between the housing case 1 and the impeller base 2 so as not to be exposed to an operating range of the vane body 3 the torsion spring 5 is spirally wound and in an embodiment thereof the torsion spring 5 is singly wound and the force thereof is determined appropriately by taking into consideration a wire diameter of.