a stapler apparatus includes a fixed frame , a movable frame pivotably mounted to the fixed frame , a staple driving device , and a staple bending device to bend the staples . a drive mechanism reciprocally moves the movable frame with respect to the fixed frame to clamp a stack of sheets between the fixed frame and the movable frame . a transmission member is movably supported on the movable frame without any connection utilizing an elongated slot . an urging member is disposed to bias the transmission member with respect to the movable frame , the transmission member being coupled to the drive mechanism such that the drive mechanism engages and moves the transmission member with respect to the movable frame , thereby overcoming the bias of the urging member , to compensate for a stack height of the stack of sheets clamped between the fixed frame and the movable frame .