A movable-blade operation system for a hydraulic machine, for adjusting opening of movable blades (14, 16) that rotate together with a rotational shaft of the hydraulic machine, the system comprising: an oil hydraulic cylinder installed within the rotational shaft, the oil hydraulic cylinder including a piston coupled to the movable blades (14, 16), and including a first cylinder chamber and a second cylinder chamber that are defined by the piston; a bidirectional pump configured to selectively feed pressurized hydraulic oil to one of the first cylinder chamber and the second cylinder chamber; a pump drive motor configured to drive the bidirectional pump; a control unit configured to control the pump drive motor, an oil head installed in the hydraulic machine and coupling the rotational shaft rotatably, the hydraulic oil to be fed from the bidirectional pump to the first cylinder chamber and the second cylinder chamber flowing through the oil head; and a sump tank configured to store hydraulic oil to be fed to the first cylinder chamber and the second cylinder chamber, wherein the bidirectional pump, the pump drive motor, the control unit, and the sump tank are installed outside the hydraulic machine, wherein the bidirectional pump and the oil head are coupled by a first pipe communicating with the first cylinder chamber and by a second pipe communicating with the second cylinder chamber, wherein the first pipe and the second pipe are connected by a bypass line (83, 84, 85), characterized in that the bypass line (83, 84, 85) communicates with a supply line extending from the sump tank, wherein a first valve (86a, 87a, 88a) controlling the flow of the hydraulic oil between the first pipe and the sump tank is provided on the side of the first pipe with respect to a communication point with the supply line in the bypass line (83, 84, 85), and a second valve (86b, 87b, 88b) controlling the flow of the hydraulic oil between the second pipe and the sump tank is provided on the side of the second pipe with respect to a communication point with the supply line in the bypass line (83, 84, 85). an oil hydraulic cylinder installed within the rotational shaft, the oil hydraulic cylinder including a piston coupled to the movable blades (14, 16), and including a first cylinder chamber and a second cylinder chamber that are defined by the piston; a bidirectional pump configured to selectively feed pressurized hydraulic oil to one of the first cylinder chamber and the second cylinder chamber; a pump drive motor configured to drive the bidirectional pump; a control unit configured to control the pump drive motor, an oil head installed in the hydraulic machine and coupling the rotational shaft rotatably, the hydraulic oil to be fed from the bidirectional pump to the first cylinder chamber and the second cylinder chamber flowing through the oil head; and a sump tank configured to store hydraulic oil to be fed to the first cylinder chamber and the second cylinder chamber, wherein the bidirectional pump, the pump drive motor, the control unit, and the sump tank are installed outside the hydraulic machine, wherein the bidirectional pump and the oil head are coupled by a first pipe communicating with the first cylinder chamber and by a second pipe communicating with the second cylinder chamber, wherein the first pipe and the second pipe are connected by a bypass line (83, 84, 85), characterized in that the bypass line (83, 84, 85) communicates with a supply line extending from the sump tank, wherein a first valve (86a, 87a, 88a) controlling the flow of the hydraulic oil between the first pipe and the sump tank is provided on the side of the first pipe with respect to a communication point with the supply line in the bypass line (83, 84, 85), and a second valve (86b, 87b, 88b) controlling the flow of the hydraulic oil between the second pipe and the sump tank is provided on the side of the second pipe with respect to a communication point with the supply line in the bypass line (83, 84, 85).