By performing preliminary scanning, a document identification unit determines whether a document is a dark document, such as a negative film, that requires excess exposure, or an ordinary document that does not require excess exposure. When it is determined that the document does not require excess exposure, an exposure time setting unit sets an exposure time for a line sensor. Then, in consonance with the designated exposure time, the line sensor initiates the reading of a black reference. Thereafter, in accordance with an excess exposure time that is designated, a black reference data preparation unit prepares black reference data. Since the black reference is read during the same period as the excess exposure time and the black reference data is prepared, the detailed tones of the dark portion of the document can be expressed, and the quality of the image can be improved.