Some image reading devices performs shading correction on read image data which is read from the original document by an image reading unit, in order to suppress difference in performance of the plurality of pixels. In order to execute shading correction, prior to reading the original document, it is necessary to acquire white reference values and black reference values to be used in the shading correction. However, if these values are generated before the original document is read, starting of reading the original document is delayed. For this reason, Japanese Patent Application Publication No. 2005-94135 discloses that the reading of the original document can be started quickly by generating only the white reference data before the original document is read, and using fixed black reference values stored in a storage unit beforehand.