An apparatus for detecting a medium image, the apparatus comprising: an image sensor comprising a light emitting unit (130, 171) configured to emit light toward a medium transferred within a range of a width of a transferring path, said transferring path being defined by a pair of reference guides (161, 162), and a light receiving unit (140, 172) configured to receive the light emitted from the light emitting unit (130, 171); a reference medium (150, 180) disposed at a position that is deviated from the range of the width of the transferring path, said position being configured to allow the image sensor to obtain a real-time image of the reference medium, whereby the image sensor is configured to obtain each of an image of the transferred medium and the real-time image of the reference medium; and a control unit; said apparatus being characterized in that: said control unit is configured to compare the real-time image of the reference medium (150, 180) to a reference image of the reference medium (150, 180), which is previously stored in a memory, and to compensate, on the basis of a comparison result of the real-time image and the reference image of the reference medium (150, 180), the image of the transferred medium, the light receiving unit (140, 172) is configured to obtain an image of the reference medium (150, 180) that is fixed in position, the image sensor comprises an extension part (131, 141, 173) extending outward from the transferring path, and the reference medium (150, 180) is disposed to face the extension part (131, 141, 173). an image sensor comprising a light emitting unit (130, 171) configured to emit light toward a medium transferred within a range of a width of a transferring path, said transferring path being defined by a pair of reference guides (161, 162), and a light receiving unit (140, 172) configured to receive the light emitted from the light emitting unit (130, 171); a reference medium (150, 180) disposed at a position that is deviated from the range of the width of the transferring path, said position being configured to allow the image sensor to obtain a real-time image of the reference medium, whereby the image sensor is configured to obtain each of an image of the transferred medium and the real-time image of the reference medium; and a control unit; said apparatus being characterized in that: said control unit is configured to compare the real-time image of the reference medium (150, 180) to a reference image of the reference medium (150, 180), which is previously stored in a memory, and to compensate, on the basis of a comparison result of the real-time image and the reference image of the reference medium (150, 180), the image of the transferred medium, the light receiving unit (140, 172) is configured to obtain an image of the reference medium (150, 180) that is fixed in position, the image sensor comprises an extension part (131, 141, 173) extending outward from the transferring path, and the reference medium (150, 180) is disposed to face the extension part (131, 141, 173). said control unit is configured to compare the real-time image of the reference medium (150, 180) to a reference image of the reference medium (150, 180), which is previously stored in a memory, and to compensate, on the basis of a comparison result of the real-time image and the reference image of the reference medium (150, 180), the image of the transferred medium, the light receiving unit (140, 172) is configured to obtain an image of the reference medium (150, 180) that is fixed in position, the image sensor comprises an extension part (131, 141, 173) extending outward from the transferring path, and the reference medium (150, 180) is disposed to face the extension part (131, 141, 173).