An image reading device includes a light source arranged below a document table glass and irradiating light to a document on the document table glass, a carriage that holds the light source and is movable in a sub-scanning direction perpendicular to a main scanning direction, a photoelectric conversion unit that receives the light irradiated by the light source and reflected from the document, a pair of rail members that extend in the sub-scanning direction while being arranged spaced apart from each other in the main scanning direction and support the carriage so as to be movable in the sub-scanning direction, a rotatable shaft, and an eccentric cam that is fixed to the shaft, has a peripheral surface abutting at least one predetermined place of the pair of rail members, and rotates together with the shaft to displace the predetermined place of the rail members in a vertical direction.