Abstract: A positioning staple case of a stacker includes a gun air-firing mechanism at one end thereof. The staple case is consisted of left and right staple cases. The invention is characterized that, the left staple case has a plurality of pressing strip and a lower side rod disposed on an inner lower side of the left staple case. Using the aforesaid structure, staples having different depths are rested on an upper portion of the lower side rod, and an upper portion of the staples is pressed against a lower portion of a specific pressing strip having a suitable depth for the staples, with the pressing strips located at a highermost position being pushed into the channels using the staples, thereby pressing against staples having different depths in a steady manner.