To prevent any loss in pressure so as to secure smooth workability of a hydraulic device by carrying out oil return via the first oil passage or the second oil passage opened at the time of cylinder operation.

When a pilot pressure makes its action via a line PiA by means of a remote control valve RV, a control valve C is moved in a position II and also it acts on a selector valve 7 to open an oil passage. Accordingly, an oil passage switching valve 3 releases an oil passage 1 and the large chamber oil of a boom cylinder A is fed back to a tank T via a relief valve R1. And also, when the pilot pressure makes its action via a line PiB by means of the remote control valve RV, the control valve C is moved in a position I and also it acts on a selector valve 8 to open an oil passage 6. Accordingly, an oil passage switching valve 4 releases an oil passage 2 and the small chamber oil of the boom cylinder A returns to the tank T via a relief valve R3.