1. An apparatus for detecting a medium image, the apparatus comprising: an image sensor comprising a light emitting unit to emit light toward a medium transferred along a transferring path and a light receiving unit to receive the light emitted from the light emitting unit; a reference medium disposed at a position that is deviated from the transferring path to allow the image sensor to obtain an image; and a control unit to compensate the image of the medium obtained from the image sensor by using the image of the reference medium obtained from the image sensor.