An image reading device includes first glass, an adjacent portion adjacent to the first glass, a reading unit, and a position reference portion. The first glass enables an document to be placed on a front surface side of the document. The reading unit is capable of being in contact with at least one of the adjacent portion and the first glass and reads, from a back surface side of the first glass, the document placed on the first glass by using a sensor while moving from the adjacent portion side in a movement direction. The position reference portion is disposed on the front surface side of the first glass and serves as a reference of a position of the reading unit in the movement direction. When the position reference portion is read by the reading unit, the reading unit is in contact with the adjacent portion and the first glass.