PURPOSE:To prevent the cavitation of a capacity variable pump by providing a switch-over valve within a communication pipe and an on-off valve within a connection pipe between an actuator and the capacity variable pump so that a sufficient amount of oil can be supplied to the pump. CONSTITUTION:Within the communication pipes 21 and 22 is provided the switch- over valve 23 and between an oil pressure motor 9 and the capacity variable pump 8 is provided the on-off valve 20. When the on-off valve 20 opened and the switch- over valve 23 is in position A, the oil pressure motor 9 is driven by the oil discharged out of the capacity variable pump 8. To the contrary, when the on-off valve 20 is closed and the switch-over valve 23 is in position B, the discharged oil is supplied to the connection pipe 5a via the communication pipe 21, thereby increasing the operation speed of an oil pressure cylinder 4. The oil pressure cylinder 4 is driven by the total amount of discharged oil from an oil pressure pump 2 and the capacity variable pump 8. Therefore, the returning oil into the connection pipe 6a is sufficient enough and hence the discharge port 8b of the capacity variable pump 8 is supplied with a sufficient amount of oil.
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