To completely dispense with the supply of lubricating oil without generating lowering of lubricating performance and seizure caused by the shortage of lubricating oil by providing an oil feeding circuit for leading operating oil fed to a hydraulic cylinder, as lubricating oil to oil retaining bushes.

In the case of the oil content of oil-retaining bushes 8, 14 being reduced, operating oil is fed to the interior and sliding faces of the oil-retaining bushes 8, 14 from oil sumps 9, 16 by a capillary phenomenon. Lubricating oil consumed at sliding parts can therefore be always supplied, sufficient lubricating oil always exists on the sliding faces so as to prevent wear of the oil-retaining bushes 8, 14 and a pin and to prolong the service life of bearings 5, 11 and pin. Since the oil-retaining bushes 8, 14 are always communicated with a bottom chamber (a) through oil feeding holes 10, 15, the oil-retaining bushes 8, 14 are always fed with operating oil so as to dispense with lubricating oil feeding work. Since the supply quantity of lubricating oil is increased in proportion to oil pressure of operating oil of a hydraulic cylinder, there is no risk of generating shortage of lubricating oil caused by the increase of load.