PURPOSE: To convert gravity acting on the flow of fluid into kinetic energy by letting the inside of a pipe in which fluid flows, be so constituted that the energy of fluid flow can be retroactive.

CONSTITUTION: An inflow section 9 provided with a water passage the cross sectional area of flow of which is contracted inwardly from an inflow port 2. A front section flow rate maintaining pressure water feeding means 4 supplies both the inflow/outflow loss compensation pressure and the dynamic pressure of a water source V2 for water flow in the vicinity of a draining port 3, to flowing water within the inside of a penstock 1, wherein the inflow/ outflow loss compensation pressure compensates both the whole of an inflow loss in pressure taking place in the specified flow rate of flowing water in a turbine driving energy system between the inflow port 2 and the outlet of an axial flow turbine 6, and the whole of an outflow loss in pressure taking place in the specified flow rate of flowing water in a flow rate maintaining energy system between the outlet of the axial flow turbine 6 and the draining port 3, to zero. A rear section flow rate maintaining pressure water feeding means 8 which is interposed between the axial flow turbine 6 and the draining port 3, supplies rear section flow rate maintaining supplemental kinetic energy to water flow within the inside of the penstock 1. An outflow section 10 is also provided, which is connected to the rear section flow rate maintaining pressure water feeding means 8, and gradually increases the cross sectional area of flow.