A binder unit, for binding a sheet stack having a mounting hole, includes an elongated binding body and two fastening arms. The binding body has a first binding portion biasing against a top side of the sheet stack at an edge portion thereof, a second binding portion biasing against a bottom side of the sheet stack, and a guiding slot aligning with the mounting hole. The fastening arms are extended from a first edge of the binding body to detachably engage with the second binding portion of the binding body at a position that the fastening arms are slidably coupled within the guiding slot through the mounting hole, wherein the fastening arms are bent for resting on the bottom side of the sheet stack to reinforce the edge portion of the sheet stack being securely sandwiched between the first and second binding portions of the binding body.