PURPOSE: To surely prevent the loosening of a nut fixing a piston to a piston rod of a hydraulic cylinder.

CONSTITUTION: A piston 11 inserted into a piston fitting part 2a of a piston rod 2 is clamped to this piston rod 2 by way of screwing a nut 12 in an external thread part 2b formed in a rear end of this piston fitting part 2a. More than one of recesses are formed in a rear end edge of the nut 12, and if the nut clamped tight with the specified torque, an end part of a platelike engaged member 14 is engaged with this recess while the engaged member 14 itself is locked to an end of the piston rod 2 by means of welding. With this constitution, any possible loosening of the nut engaging with the engaged member 14 via the recess is prevented by this engaged member 14.