To prevent the loosening of a free lock-lock nut and evade a lock abnormal operation of the free lock beforehand.

A free lock 16 is arranged between a lower frame 11 and an upper frame 12 by performing a pin coupling with the lower frame 11 for a body 17 and performing a thread coupling with an end bracket 21 of the upper frame 12 for a piston rod 18. The thread coupling is performed for a lock nut 20 on the piston rod 18. At an adjusting location which makes the projecting amount of a push rod 19 constant, the piston rod 18 is locked to the end bracket 21. A nut cover 31 has a cylindrical outer peripheral surface, and the cover 31 is covered with the lock nut 20 and is latched.