PURPOSE: To make it possible to ensure the detection of the size of a manuscript accurately, by providing a light polarizing filter, which transmits or shields only a specified polarization component of the reflected light of light emitted from a light source, between the light source and a light receiving element.

CONSTITUTION: A light emitting diode 2 and a photo-transistor 3 are fixed in a metal or plastic case so that the optical axes form a required angle. Light emitted from the light emitting diode 2 is reflected by a glass plate 5 or the like, which is close to a bottom surface opening 4 of the case 1. The diode 2 and the transistor 3 are arranged so that the reflected light is inputted to the photo- transistor 3. A light polarizing filter 6, which transmits only the specified polarization component of said reflected light, is provided in front of said photo- transistor 3. In this constitution, the size of a manuscript, which is mounted on a platen, can be detected accurately and positively, by adding a small number of parts to a conventional reflection type photosensor.