To ensure recovery of deteriorating working oil and bleed of air and to relax the pressure resisting condition of a piping to recover working oil.

In a hydraulic circuit, when high oil pressure generated by an oil pressure generating mechanism 2 is fed to a hydraulically-operating mechanism 3 through a check valve 34 and a pressure feed pipe 13, the piston 42 of a pilot hydraulically-operated on-off valve 30 is moved to the right to close an on-off valve 54 and the hydraulically-operated mechanism 3 performs ordinary hydraulic operation. When the generating oil pressure of the oil pressure generating mechanism 2 is decreased, a check valve 34 is closed and an on-off valve 54 is opened. Deteriorating working oil having a possibility to contain air bubbles is recovered in a low pressure state to a tank 4 from the hydraulically-operated mechanism 3 through a depressurization return pipe 26 and the on-off valve 54.