An image reading apparatus includes image reader reading an image of a manuscript which is placed on a manuscript platen, and size detector detecting a size of the manuscript, wherein the image reader slides inside a chassis, which is arranged below the manuscript platen, in parallel to the manuscript platen and reads the image of the manuscript, wherein the size detector includes two light sensors each of which is composed of a pair of light emitter emitting light toward the manuscript platen from a position under the manuscript platen, and light receiver receiving reflection light which is reflected by the manuscript, wherein the light emitter and the light receiver are mounted on one substrate so that each line connecting the light emitter with the light receiver of the light sensor may be on one straight line, and wherein the size detector is arranged at one internal side area of the chassis, which does not overlap with an area through which the image reader slides, so that the line connecting the light emitter with the light receiver may be parallel to a sliding direction of the image reader.