transverse rod 4 for fixing the mounting rings 5 on transverse rod 4 referring to fig4 the mounting ring 5 has an opening 52 and an upper edge 53 and a fastening arm 54 is situated on the top and bottom of the opening 52 a small slot 541 is located at the end of the fastening arm 54 the top of add on card 3 can be inserted into the small sot 541 a screw 6 passes through the upper edge 53 and the fastening arm 54 to fasten the mounting ring 5 firmly on transverse rod 4 the fastening arm 54 is on the top of the add on card 3 and presses it firmly from moving and popping up based on above description the transverse rod 4 is installed on the rear of computer case 1 and above the expansion slots 21 of the mainboard 2 the mounting ring 5 wraps around the transverse rod 4 by pulling the opening 52 wider the mounting ring 5 is moved to top of an add on card 3 press the fastening arm 54 to the add on card 3 and inlay the small slot 541 to the top of add on card 3 pass the screw 6 over the upper edge 53 to the fastening arm 54 to fasten the mounting ring 5 on the transverse rod 4 accordingly the add on card 3 can be fixed for prevention from bad contact.