PROBLEM TO BE SOLVED: To provide a method for forming black reference data at the time of overexposure in which the accuracy of black reference data and the image quality are enhanced. SOLUTION: By preliminary scanning, a document deciding section 42 makes a decision whether a document is a dark document, e.g. a negative film requiring overexposure or a normal document requiring no overexposure. If the document requires overexposure, an exposure time setting section 43 sets the exposure time of a line sensor. The line sensor performs reading of a black reference according to the set exposure time. A black reference data generating section 44 generates black reference data from the output at the time of reading a black reference which is read by the line sensor depending on a set excess exposure time. When a black reference is read at the same time as the excess exposure time and then black reference data is generated, gray scale at a dark part of a document being read out can be represented in detail and the image quality of a read image is enhanced.
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