An optical unit of an image reading device has the center of gravity in a position toward a first sub scanning direction, relative to the middle point of the optical unit in terms of sub scanning directions. An engagement part is provided in a position toward a first main scanning direction relative to a sliding part. A plurality of pairs of pinching parts include a first pair provided in a position toward the first sub scanning direction and a second pair provided in a position toward a second sub scanning direction. Of the two pinching parts in the second pair, the pinching part provided in a position toward the first main scanning direction is biased by a first biasing member toward a rail part side, and the pinching part provided in a position toward a second main scanning direction is not biased by the first biasing member.