An image reading apparatus including a light source for illuminating an original, a plurality of mirrors for reflecting a light beam from the original illuminated by the light source, a reading device, an imaging optical system for imaging a light beam reflected by the mirrors, upon the reading device, and a scanning optical system unit which integrally accommodates theses components therein and moves in a sub-scan direction to read imagewise information of the original, wherein, in a sub-scan section, one mirror optically closest to the surface of the original is disposed in an end portion of the scanning optical system unit in the sub-scan direction, as compared with the other mirrors, and wherein the central portion of the reflection surface of one mirror optically closest to the original is disposed in a space between the central portion of the reflection surface of a mirror optically closest to the light entrance surface of the imaging optical system and central portions reflection surfaces of other mirrors.