An image reading device includes a reading member and a skew detector. The reading member reads an image of a passing document at a read position preset in a document transport path. The skew detector detects a skew of the document by selecting one of a first skew detection mode and a second skew detection mode based on information that specifies a method for detecting the skew of the document. The first skew detection mode is a mode in which the skew of the document relative to a transport direction is detected based on an image of a leading-edge section, in the transport direction, read by the reading member. The second skew detection mode is a mode in which the skew of the document relative to the transport direction is detected based on an entire image read by the reading member.