A controller unit controls an image processing apparatus that includes a scanner unit that acquires image data by scanning an original. The controller unit includes a scanning characteristic correcting unit that converts the image data acquired by the scanner unit into normalized image data, an expanded I/F unit to which a controller image processing unit can be detachably attached to perform an image reducing operation on the normalized image data, and a basic image processing unit that, when the controller image processing unit is not attached to the expanded I/F unit, performs a basic image reducing operation on the normalized image data.