A scan guide upon image reading by an image reading apparatus having a driving mechanism constructed by a rack and a pinion gear is stably performed. The pinion gear is provided on a downstream side of two contact portions which are in contact with a guide rail of the reading scan. A straight line drawn in a pressure angle direction from a contact point of the rack and the pinion gear upon reading and scanning passes between the two contact portions with the guide rail, so that a stable positional relation between a driving force of the gear and the contact portions is obtained. The stable guiding can be performed.