disengage from the corresponding through holes 242 of the blocking plate 24 the button 6 is released the holding member 5 is restored to make the holding plate 51 tightly abut against the sidewall 12 of the supporting bracket 1 the handle 248 is released the resilient member 7 is restored to pull the slide bracket 20 backwards until the positioning pins 40 abut against the forward sides 5152 of the corresponding retaining portions 515 of the holding plate 51 therefore the latch member 3 is kept in an unlocked position ready for engaging with the rack post 90 referring to fig4 a the slide rail 80 is handled to make the inserting pins 162 insert into the corresponding through holes 92 of the rack post 90 until the shoulders 1625 of the inserting pins 162 abut against a rear surface of the rack post 90 the button 6 is pushed to move the retaining portions 515 of the holding plate 51 away from the sidewall 12 of the supporting bracket 1 to disengage the holding member 5 from the positioning pins 40 the resilient member 7 is restored to slide the slide bracket 20 backwards until the blocking plate 24 abuts against a front surface of the rack post 90 to tightly sandwich the rack post 90 between the blocking plate 24 and the shoulders 1625 of the inserting pins 162 at the same time the positioning pins 40 slide with the slide bracket 20 along the corresponding first slide slots 122 of the supporting bracket 1 to approach to the rear ends of the first slide slots 122 the button 6 is released the holding member 5 is restored the retaining portions 515 of the holding plate 51 move towards.