PURPOSE: To make a change in a delivery amount of pressure oil according to the loading status of a piston rod by providing a partial combination consisting of a piston and a cylinder keeping a preset clearance between the piston and the cylinder.

CONSTITUTION: Among the combinations consisting of plural pistons 17aW17c and cylinders 16aW16c, a clearance 20 with a preset value is provided between the piston and the cylinder in a certain combination. In the case of no load on an operation mechanism, a comparatively high forward speed of the piston rod can be obtained, because a large amount of pressure oil can be press fed to a hydraulic chamber by the viscosity of oil in the pistons 16a, 16b having a clearance 20. In the case of loading, a press fed amount of the pressure oil to the hydraulic chamber can be reduced, because the oil is released from a clearance 20 to the side of the oil section hole 18 by the reaction force of the operation mechanism.