Image reading device capable of suppressing situation where image read from double-page document sheet becomes image unintended by user, image forming apparatus, and image reading method

An image reading method executed in an image reading device including a document sheet placing portion on which a document sheet is placed and a cover portion provided while being capable of changing its attitude between a closed attitude in which the cover portion covers the document sheet placing portion and an opened attitude in which the cover portion exposes the document sheet placing portion, the image reading method comprising:

• an acceptance step of accepting a designation operation for designating a size of the document sheet placed on the document sheet placing portion;
• a determination step of determining whether or not the size of the document sheet has been designated by the designation operation when an instruction to read an image of the document sheet placed on the document sheet placing portion is input and the cover portion is in the opened attitude;
• and
• an output step of executing, when it is determined by the determination step that the size of the document sheet has not been designated, first output processing for reading an image of a predetermined first region in the document sheet placing portion, correcting a tilt of each of page images included in a first image obtained by the reading, and clipping a document sheet region from the corrected first image and outputting the document sheet region, and executing, when it is determined by the determination step that the size of the document sheet has been designated, second output processing for reading an image of a second region in the document sheet placing portion, that corresponds to the size designated by the designation operation, and outputting a second image obtained by the reading.