Certain embodiments provide a stapling unit including a stapler, a processing tray, a stapler moving mechanism, a sensor configured to scan sheets in a sheet width direction, and a controller configured to drive the stapler. The controller measures sheet width of the sheets according to an output of the sensor. The controller calculates a center position between other two sheet edges each orthogonal to a sheet edge where the stapler drives staples into stapling positions. The controller allocates the stapling positions in the two places with reference to the center position. The controller moves the stapler moving mechanism to the stapling positions.