PURPOSE: To surely prevent a slide from dropping down by providing a lock cylinder to engage a male tooth of the top end of a rod fixed on the slide and to lock the rod when the hydraulic pressure is reduced from a prescribed value.

CONSTITUTION: The position that the top end of a rod top end metal 15 is entered to the tooth groove of a rack 11 is made with a direction switching valve 20, next the top end of the rod top end metal 15 is made to engage with the tooth groove of the rack 11 with a direction switching valve 24 and the direction switching valve 20 is made at a center loop position. The rack 11 is moved in the up and down direction against the advance of the rod top end metal 15 and fitted to the tooth groove surely, but in this state the rack is made in the supporting state to the slide 4 through a lock cylinder 32. Caused on the slope of the upper side of the tooth groove of the rack 11, the force to release fitting with the rack 11 is applied to the rod top end metal 15, but because the cylinder rod of the lock cylinder 32 is fixed with a rod part 33 even if the hydraulic pressure is reduced from the prescribed value, the drop of the slide 4 can be prevented surely.