An image reading apparatus includes an original carriage for carrying an original; a reading element for reading an original image with relative movement therebetween, the reading element having a configuration elongated in a direction crossing with a direction of the movement; a supporting member for supporting the image reading unit; moving means for driving the supporting member to impart the relative movement; a first positioning portion for determining a position, in the longitudinal direction, of the reading element relative to the supporting member; and a second positioning portion for determining a position of the reading element in the moving direction, the second positioning portion supporting the reading element while permitting movement thereof in the longitudinal direction relative to the supporting member.