PURPOSE: To make the extent of absorption power alterable in keeping an impeller revolution constant intact by installing a diaphragm, capable of expansion or contraction, in space between stator blades themselves and also a diaphragm expansion control means, injecting and regulating a capacity variable fluid, in this diaphragm respectively.

CONSTITUTION: When the extent of absorption power in a hydraulic brake is reduced, a pressure chamber 61 is pressurized with a capacity variable fluid by a pressure device 6, and if it has cane to the specified pressure, a valve 63 is closed. At this time, a diaphragm 5 expands, while a filling quantity in a braking fluid such as water or the like in a blade passage lying between an impeller 1 and a stator 2a is decreased, and thereby the absorption power as the hydraulic brake is abated. When this absorption power is made larger, the valve 63 is opened, lowering the extent of pressure in the pressure chamber 61 up to the specified pressure, and then this valve 63 is closed. When the diaphragm 5 withers, and the filling quantity of the braking fluid in the blade passage between the impeller 1 and the stator 2a is increased the other way, and thus the absorption power grows. With this constitution, the absorption power is thus alterable in keeping a revolving speed of the impeller constant intact.