An image reading device comprises: a feeder that feeds a document along with a predetermined path; a first reading part that reads a first side of the document at a first position in the path and creates a first image; a second reading part that reads a second side of the document at a second position which is located posterior to the first position in the path and creates a second image; a synthesizing part that lays the first image and the second image on top of one another and creates a composite image; and a detecting part that detects a characteristic pattern of a specific document from the composite image created by the synthesizing part.