flange 32 of the cap 3 is stopped at the bottom side of the top coupling flange 13 of the barrel 1 thus the plate member fastener is assembled and the screw member 2 is prohibited from escaping from the barrel 1 when fastening the first metal plate member 5 to a second metal plate member 6 attach the first metal plate member 5 to the second metal plate member 6 to keep the mounting through hole 51 of the first metal plate member 5 in alignment with a screw hole 61 of the second metal plate member 6 and then force the cap 3 downwards and rotate the cap 3 to drive the thread 231 of the screw member 2 into the screw hole 61 of the second metal plate member 6 and then use a hand tool for example screwdriver and attach the hand tool to the tool groove 24 of the screw member 2 and rotate the hand tool to tighten up the connection between the screw member 2 and the second metal plate member 6 thus the first metal plate member 5 and the second metal plate member 6 are firmly secured together by the plate member fastener further when wishing to remove the first metal plate member 5 from the second metal plate member 6 rotate the cap 3 in the reversed direction to disengage the screw member 2 from the screw hole 61 of the second metal plate member 6 at this time the first metal plate member 5 is unlocked from the second metal plate member 6 and can be directly removed from the second metal plate member 6 further the barrel 1 is.